Canonical mTOR signaling supports complete fin regeneration
The mechanistic target of rapamycin (mTOR) pathway is a deeply conserved regulator of cellular growth, metabolism, and tissue repair. In vertebrates, mTOR activity is broadly required for wound healing and regeneration. Recent work in salamanders identified unique amino acid expansions within the mTOR protein. These expansions are thought to confer hypersensitive kinase activity, which was proposed to support rapid translational activation and exceptional limb regeneration capacity. This has raised the possibility that canonical, mammalian-like mTOR signaling may be insufficient to support regeneration of complex appendages. Here, we test this hypothesis by investigating fin regeneration in the Senegal bichir (Polypterus senegalus), a ray-finned fish capable of fully regenerating fins containing bone, cartilage, muscle, and connective tissue with cellular complexity comparable to tetrapod limbs. We show that fin amputation triggers rapid activation of canonical mTOR signaling, and pharmacological inhibition of mTOR with rapamycin abolishes fin regeneration despite successful wound closure. Single-nucleus RNA sequencing (snRNA-seq) across multiple regenerative stages reveals that mTOR pathway components are selectively upregulated in proliferative epidermal and connective tissue cells, and myeloid cell populations. Rather than causing a global shutdown of gene expression, mTOR inhibition specifically dampened the induction of translational machinery and glycolytic programs, two canonical downstream outputs of mTOR signaling. In addition, myeloid cells exhibited pronounced sensitivity to mTOR inhibition, showing attenuated immune-competent and pro-regenerative programs such as interferon-associated signaling, antigen processing, and innate immune coordination. We propose that salamander-specific mTOR hypersensitivity represents an evolutionary acceleration of an ancestral regenerative program, rather than a prerequisite for limb regeneration. Significance StatementThe ability to regenerate complex appendages varies widely among vertebrates, yet the molecular basis of this variation remains poorly understood. Salamanders harbor a uniquely hypersensitive form of mTOR that has been proposed to be essential for limb regeneration. Using Polypterus fin regeneration as a model system, we demonstrate that a canonical vertebrate mTOR protein is compatible with regeneration of a complex, limb-like appendage. Our results show that mTOR acts through selective metabolic, translational, and immune programs. These findings suggest that salamander-specific mTOR hypersensitivity represents a derived enhancement rather than a condition for appendage regeneration and provide insight into the evolutionary foundations of regenerative competence.