A Rapid In Vivo Bioassay for Developmentally Active Enhancers

A potential regulatory sequence (PRS) can be identified by several approaches including conservation analysis and transcription factor-specific chromatin immunoprecipitation followed by next generation sequencing (TF ChIP-seq). TF ChIP-seq generates a large data set (TF-regulatome) that includes PRSs and background interactions. These approaches require secondary low throughput validation in model systems that replicate the temporal and spatial specificity of the PRS.

In this report, a protocol is described to rapidly validate activity of PRSs in their developmental context. Following selection of candidate PRSs, the associated sequences are isolated, cloned into a GFP reporter construct, and then transfected into chick embryos. For PRSs that are anticipated to be functional during early development, from gastrulation through early organogenesis, whole embryo electroporation (EP) is recommended. With this technique transfection, incubation and monitoring occurs ex ovo during early gastrulation. The entire embryo is transfected and as structures form, they can be monitored live for PRS activity. This ex ovo approach supports development up to early limb bud outgrowth.

If the functional activity of a PRS is expected to be later in development or if the investigation is to ascertain its activity during the development of a specific organ, targeted regional electroporation (TREP) is used. This approach requires injection of construct DNA directly into the developing organ, precursor or adjacent cavity prior to electroporation. An approach to transfect the presumptive limb bud using TREP is also outlined and illustrated. TREP supports a rapid organ-specific validation of a PRS' activity. Both techniques can also be used to investigate regulatory domains by site-directed mutagenesis of the PRS-reporter construct.

The chick as a bioassay offers a rapid effective tool for PRS validation, functional domain determination, and further characterization of the regulatory events responsible for development.