Expression of the bacterial enzyme nitroreductase (NTR) does not require light but instead uses the pro-drug uptake of the expressing tissue to elicit induction of cell death through enzyme-mediated pro-drug conversion into a p53-independent DNA-damaging compound ( Cui et al., 1999 Djeha et al., 2005). However, its use is restricted to tissues accessible by the often shallow light penetration. Cell type-specific or subcellular-specific expression of the phototoxic fluorescent proteins KillerRed (KR) ( Bulina et al., 2006) or SuperNova ( Takemoto et al., 2013) can be used to generate radicals upon excitation, which either results in cell death or in destruction of the targeted organelle ( Teh et al., 2010 de Anda et al., 2010 Korzh et al., 2011). To obtain spatial and temporal control over the cell type to be ablated, and to avoid collateral tissue damage that is associated with invasive ablation techniques, inducible genetic cell ablation methods were developed. Working with zebrafish embryos, one can combine in vivo cell ablation with multicolor live-imaging techniques and numerous forms of genetic or pharmaceutical manipulations due to the aquatic lifestyle, small size, external development and transparency of these embryos ( Lieschke and Currie, 2007 Weber and Köster, 2013). In vivo cell ablation is a useful research method with which to study the development, function, interaction and regeneration of cells or tissues in a multicellular context. Thus, induction of apoptosis through targeted activation of caspase by tamoxifen (ATTAC TM) further expands the repertoire of genetic tools for conditional interrogation of cellular functions. We observed synchronous cell death autonomous to the PC population and phagocytosing microglia in the cerebellum, reminiscent of developmental apoptosis in the forebrain. Incubation of larvae in tamoxifen for 8 h activated endogenous Caspase 3 and cell death, whereas incubation for 16 h led to the near-complete loss of PCs by apoptosis. Here, we prove its suitability in vivo by monitoring the ablation of cerebellar Purkinje cells (PCs) in transgenic zebrafish that co-express the inducible caspase and a fluorescent reporter. We have recently established a novel inducible genetic cell ablation system based on tamoxifen-inducible Caspase 8 activity, thereby exploiting mechanisms of cell death intrinsic to most cell types. Existing cell ablation methods are either invasive or they rely on the cellular expression of prokaryotic enzymes and the use of antibiotic drugs as cell death-inducing compounds. The zebrafish is a well-established model organism in which to study in vivo mechanisms of cell communication, differentiation and function.
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