SYNAPSE-SPECIFIC AND NEUREGULIN-INDUCED TRANSCRIPTION REQUIRE AN ETS SITE THAT BINDS GABP-ALPHA GABP-BETA/

Localization of acetylcholine receptors (AChRs) to neuromuscular synap ses is mediated by multiple pathways. Agrin, which is the signal for o ne pathway, stimulates a redistribution of previously unlocalized AChR s to synaptic sites. The signal for a second pathway is not known, but this signal stimulates selective transcription of AChR genes in myofi ber nuclei located near the synaptic site. Neuregulin (NRG) is a good candidate for the extracellular signal that induces synapse-specific g ene expression, since NRG is concentrated at synaptic sites and activa tes AChR gene expression in cultured muscle cells. Previous studies ha ve demonstrated that 181 bp of 5' flanking DNA from the AChR delta-sub unit gene are sufficient to confer synapse-specific transcription in t ransgenic mice and NRG responsiveness in cultured muscle cells, but th e critical sequences within this cis-acting regulatory region have not been identified. We transfected AChR delta-subunit-hGH gene fusions i nto a muscle cell line, and we show that a potential binding site for Ets proteins is required for NRG-induced gene expression. Furthermore, we produced transgenic mice carrying AChR delta-subunit-hGH gene fusi ons with a mutation in this NRG-response element (NRE), and we show th at this NRE is necessary for synapse-specific transcription in mice. T he NRE binds proteins in myotube nuclear extracts, and nucleotides tha t are important for NRG responsiveness are likewise critical for forma tion of the protein-DNA complex. This complex contains GABP alpha, an Ets protein, and GABP beta, a protein that lacks an Ets domain but dim erizes with GABP alpha, because formation of the protein-DNA complex i s inhibited by antibodies to either GABP alpha or GABP beta. These res ults demonstrate that synapse-specific and NRG-induced gene expression require an Ets-binding site and suggest that GABP alpha/GABP beta med iates the transcriptional response of the AChR delta-subunit gene to s ynaptic signals, including NRG.