Heat-Shock-Inducible Gene Expression Systems in Amphibians: Utility in Regeneration and Metamorphosis Studies

Amphibians exhibit two remarkable biological phenomena: regeneration and metamorphosis. The ability to regenerate damaged body parts, such as the limbs, and to remodel organs-such as tail resorption during metamorphosis-is both fascinating and enigmatic. However, until recently, it has been difficult to manipulate gene expression in amphibians after embryogenesis, hindering molecular studies of these processes. Over the past two decades, the development of a simple and reproducible gene expression method-the heat-shock-inducible system-has helped overcome this limitation. This system involves generating transgenic animals carrying gene(s) of interest under the control of a heat shock promoter, typically the hsp70 promoter, followed by heat shock treatment to induce expression. Recent advancements have enabled not only the application of heat shock to the whole body but also spatially restricted gene induction in specific cell populations. In particular, laser irradiation allows for highly precise gene activation and lineage tracing, even at the single-cell level. One current limitation of this system is unintended "leaky" gene expression in the absence of heat shock. The recent availability of an alternative inducible system, Tet-on, in amphibians holds promise for overcoming this drawback and achieving tighter control of gene expression. In this review, we discuss potential refinements to the heat-shock-inducible system-including improvements in laser irradiation techniques and optimization of heat shock promoters (e.g., hsp70 promoter)-to address current limitations, and explore how this system may become an even more powerful tool for studying regeneration and metamorphosis in amphibians.