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Neuronal activity regulates the development and maturation of excitatory and inhibitory synapses in the mammalian brain. Several recent studies have identified signalling networks within neurons that control excitatory synapse development. However, less is known about the molecular mechanisms that regulate the activity-dependent development of GABA (γ-aminobutyric acid)-releasing inhibitory synapses. Here we report the identification of a transcription factor, Npas4, that plays a role in the development of inhibitory synapses by regulating the expression of activity-dependent genes, which in turn control the number of GABA-releasing synapses that form on excitatory neurons. These findings demonstrate that the activity-dependent gene program regulates inhibitory synapse development, and suggest a new role for this program in controlling the homeostatic balance between synaptic excitation and inhibition. Balance of mind A fine balance between the numbers of excitatory and inhibitory synapses must be maintained for neuronal circuits to function. The intracellular molecular signalling pathways involved in activity-dependent formation of synapses, particularly inhibitory ones, are largely unknown. A new study has identified the transcription factor Npas4 as a 'master switch' acting in brain cells to maintain the homeostatic balance between synaptic excitation and inhibition, a balance that is thought to be disrupted in neurologic disorders such as autism, epilepsy and schizophrenia. Npas4 acts by regulating the expression of more than 200 activity-dependent genes, which in turn control the number of GABA-mediated synapses that form excitatory neurons.

Eri1 is a 3′-to-5′ exoribonuclease conserved from fission yeast to humans. Here we show that Eri1 associates with ribosomes and ribosomal RNA (rRNA). Ribosomes from Eri1 –deficient mice contain 5.8S rRNA that is aberrantly extended at its 3′ end, and Eri1, but not a catalytically inactive mutant, converts this abnormal 5.8S rRNA to the wild-type form in vitro and in cells. In human and murine cells, Eri1 localizes to the cytoplasm and nucleus, with enrichment in the nucleolus, the site of preribosome biogenesis. RNA binding residues in the Eri1 SAP and linker domains promote stable association with rRNA and thereby facilitate 5.8S rRNA 3′ end processing. Taken together, our findings indicate that Eri1 catalyzes the final trimming step in 5.8S rRNA processing, functionally and spatially connecting this regulator of RNAi with the basal translation machinery.