Two axolotl-adapted cell-ablation platforms reveal macrophage-dependent processes essential for spinal-cord and skeletal regeneration

Abstract

The axolotl (Ambystoma mexicanum) has emerged as the premier model organism for studying scarless repair and adult tissue regeneration, supported by an expanding collection of tissue-specific transgenic lines and translucent skin that enables high-quality live imaging and cell tracking. However, functional characterization of specific cell types during regeneration has been limited by the absence of validated cell-specific ablation systems. Here, we developed and rigorously compared two independent inducible genetic cell-ablation platforms -- bacterial nitroreductase (NTR 2.0) and mammalian inducible caspase-9 (iCasp9), across developmental stages, animal sizes, and administration routes using various transgenic lines and grafting approaches. The NTR 2.0 platform showed limited applicability due to drug toxicity and solubility constraints, restricting its use primarily to larval stages via immersion. In contrast, the iCasp9 system demonstrated superior efficacy across all life stages, including large adults, with multiple viable administration routes. We further validated these platforms by systematically ablating CD68+ macrophages and examined functional consequences during tail regeneration. Sustained depletion revealed essential macrophage-dependent processes despite continuous macrophage repopulation from hematopoietic reservoirs: skeletal-element regeneration was completely abolished, spinal-cord axons degenerated without recovery, and neural crest-derived cells exhibited severe disorganization. These findings establish macrophages as critical orchestrators of central and peripheral nervous-system regeneration and skeletogenesis in axolotls, while providing validated tools for cell-type-specific functional studies across the axolotl lifespan.