Calcineurin mediates homeostatic synaptic plasticity by regulating retinoic acid synthesis.

Homeostatic synaptic plasticity is a form of non-Hebbian plasticity that maintains stability of the network and fidelity for information processing in response to prolonged perturbation of network and synaptic activity. Prolonged blockade of synaptic activity decreases resting Ca(2+) levels in neurons, thereby inducing retinoic acid (RA) synthesis and RA-dependent homeostatic synaptic plasticity; however, the signal transduction pathway that links reduced Ca(2+)-levels to RA synthesis remains unknown. Here we identify the Ca(2+)-dependent protein phosphatase calcineurin (CaN) as a key regulator for RA synthesis and homeostatic synaptic plasticity. Prolonged inhibition of CaN activity promotes RA synthesis in neurons, and leads to increased excitatory and decreased inhibitory synaptic transmission. These effects of CaN inhibitors on synaptic transmission are blocked by pharmacological inhibitors of RA synthesis or acute genetic deletion of the RA receptor RARα. Thus, CaN, acting upstream of RA, plays a critical role in gating RA signaling pathway in response to synaptic activity. Moreover, activity blockade-induced homeostatic synaptic plasticity is absent in CaN knockout neurons, demonstrating the essential role of CaN in RA-dependent homeostatic synaptic plasticity. Interestingly, in GluA1 S831A and S845A knockin mice, CaN inhibitor- and RA-induced regulation of synaptic transmission is intact, suggesting that phosphorylation of GluA1 C-terminal serine residues S831 and S845 is not required for CaN inhibitor- or RA-induced homeostatic synaptic plasticity. Thus, our study uncovers an unforeseen role of CaN in postsynaptic signaling, and defines CaN as the Ca(2+)-sensing signaling molecule that mediates RA-dependent homeostatic synaptic plasticity.

Aldehyde dehydrogenase 1a1 mediates a GABA synthesis pathway in midbrain dopaminergic neurons.

Midbrain dopamine neurons are an essential component of the basal ganglia circuitry, playing key roles in the control of fine movement and reward. Recently, it has been demonstrated that γ-aminobutyric acid (GABA), the chief inhibitory neurotransmitter, is co-released by dopamine neurons. Here, we show that GABA co-release in dopamine neurons does not use the conventional GABA-synthesizing enzymes, glutamate decarboxylases GAD65 and GAD67. Our experiments reveal an evolutionarily conserved GABA synthesis pathway mediated by aldehyde dehydrogenase 1a1 (ALDH1a1). Moreover, GABA co-release is modulated by ethanol (EtOH) at concentrations seen in blood alcohol after binge drinking, and diminished ALDH1a1 leads to enhanced alcohol consumption and preference. These findings provide insights into the functional role of GABA co-release in midbrain dopamine neurons, which may be essential for reward-based behavior and addiction.

Drosophila glutamate receptor mRNA expression and mRNP particles.

The processes controlling glutamate receptor expression early in synaptogenesis are poorly understood. Here, we examine glutamate receptor (GluR) subunit mRNA expression and localization in Drosophila embryonic/larval neuromuscular junctions (NMJs). We show that postsynaptic GluR subunit gene expression is triggered by contact from the presynaptic nerve, approximately halfway through embryogenesis. After contact, GluRIIA and GluRIIB mRNA abundance rises quickly approximately 20-fold, then falls within a few hours back to very low levels. Protein abundance, however, gradually increases throughout development. At the same time that mRNA levels decrease following their initial spike, GluRIIA, GluRIIB, and GluRIIC subunit mRNA aggregates become visible in the cytoplasm of postsynaptic muscle cells. These mRNA aggregates do not colocalize with eIF4E, but nevertheless presumably represent mRNP particles of unknown function. Multiplex FISH shows that different GluR subunit mRNAs are found in different mRNPs. GluRIIC mRNPs are most common, followed by GluRIIA and then GluRIIB mRNPs. GluR mRNP density is not increased near NMJs, for any subunit; if anything, GluR mRNP density is highest away from NMJs and near nuclei. These results reveal some of the earliest events in postsynaptic development and provide a foundation for future studies of GluR mRNA biology.

Regulation of glutamate receptor subunit availability by microRNAs.

The efficacy of synaptic transmission depends, to a large extent, on postsynaptic receptor abundance. The molecular mechanisms controlling receptor abundance are poorly understood. We tested whether abundance of postsynaptic glutamate receptors (GluRs) in Drosophila neuromuscular junctions is controlled by microRNAs, and provide evidence that it is. We show here that postsynaptic knockdown of dicer-1, the endoribonuclease necessary for microRNA synthesis, leads to large increases in postsynaptic GluR subunit messenger RNA and protein. Specifically, we measured increases in GluRIIA and GluRIIB but not GluRIIC. Further, knockout of MiR-284, a microRNA predicted to bind to GluRIIA and GluRIIB but not GluRIIC, increases expression of GluRIIA and GluRIIB but not GluRIIC proportional to the number of predicted binding sites in each transcript. Most of the de-repressed GluR protein, however, does not appear to be incorporated into functional receptors, and only minor changes in synaptic strength are observed, which suggests that microRNAs primarily regulate Drosophila receptor subunit composition rather than overall receptor abundance or synaptic strength.