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Cis-regulatory integration of intrinsic and target-dependent regulators is required for terminal differentiation of Drosophila neurons

University of British Columbia

Terminal differentiation of neurons often requires retrograde signals from the target cells they innervate, which trigger neural subtype-specific gene expression upon target contact. Target-derived BMP signaling and transcription factors including the LIM-Homeodomain transcription factor Apterous are required for FMRFa neuropeptide gene expression in Drosophila Tv4 neurons. We modeled the integrative mechanism of these extrinsic and intrinsic inputs at a Tv4 neuron-specific FMRFa enhancer. We show that Tv4-specific FMRFa expression requires two separable cis-elements, a BMP-response element (BMP-RE) that binds Mad, and a homeodomain response element (HD-RE) that binds Apterous. Strikingly, we find that concatemers of these two short (~30bp) cis-elements each independently drive spatial and temporal expression appropriate for Tv4-specific FMRFa. Thus, specific and robust expression is generated from the synergy of two low-activity heterotypic cis-elements that encode the same output from distinct inputs. We further examined the timing mechanism of FMRFa initiation, which models predict would be solely based on target contact. In contrast, we find that the timed downregulation of the COUP-TF I/II nuclear receptor Seven up functions to de-repress HD-RE and BMP-RE activity immediately prior to target contact. Thus, we reveal that the active suppression of neurotransmitter identity, prior to target contact, is an innate component of the target-dependent mechanism for timed gene activation. Further examination of the FMRFa BMP-RE shows that the sequence of the Mad and Medea binding sites to be “perfectly wrong.” It displays sequence similarity to both the previously characterized BMP Silencing Element (SE) and Activating Elements (AE) but with base substitutions that should abrogate Smad binding. Biochemical and reporter construct analysis demonstrate that the FMRFa BMP-RE is an atypical activating element that is specifically attenuated in its ability to interact with the co-Smad Medea. In vivo reporter assays show that this attenuation is required to drive appropriate expression in the CNS. These findings represent only the third verified type of BMP-RE found in Drosophila leading us to call this class an AE2 element.

Text

2018-10-31

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/55193

Cell and Developmental Biology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/