Mosaic activation of the zebrafish hsp70l heat shock promoter: Implications for interpreting transgenes

The promoters and enhancers of heat shock genes, such as the 1.5-kb promoter of the zebrafish hsp70l gene, are valuable tools for temporal activation of transgenes. It has been widely purported that heat shock treatments result in ubiquitous expression of hsp70l-driven transgenes. However, variations in expression levels of transgenes are observable in many studies, making it necessary to further characterize the transcriptional activity of the hsp70l promoter.

Here, we report that the transgenic expression pattern of hsp70l-driven EGFP-LacR (a cell nucleus-localizing fusion protein of EGFP and Lac repressor) demonstrates explicitly that the hsp70l promoter is activated by heat shock in a mosaic fashion in 50-80% of embryonic cells and at even lower rates in postmitotic retinal cells in larvae. This mosaic expression is influenced by cell-cycle states and cell-type specificity. The expression rates at early embryonic stages can be increased by multiple heat shocks or cell cycle synchronization.

The mosaic activation of the hsp70l promoter does not disqualify it as a powerful temporal gene activation tool. Mosaic activation can be used as an advantage for contexts where a transgene functions cell-autonomously; however, caution and proper controls are necessary to interpret gene functions in contexts where a transgene acts non-cell-autonomously.