Transforming growth factor-β-mediated regulation of atoh1-expressing neural progenitors is involved in the generation of cerebellar granule cells in larval and adult zebrafish.

Granule cells in the cerebellum are the most numerous neurons in the vertebrate brain. They are derived from neural progenitor cells that express the proneural gene atoh1 (atoh1a, b, c in zebrafish) during early neurogenesis. In zebrafish, unlike in mammals, granule cells are continuously produced throughout life, from the larval stage to adulthood. Additionally, granule cells regenerate and replace damaged areas following injury in the adult cerebellum. However, the mechanisms underlying granule cell generation and their role in adult cerebellar regeneration remain largely unclear. In this study, using lineage tracing with the inducible DNA recombinase CreERT2, we found that granule cells differentiated from atoh1c-expressing neural progenitor cells and migrated to their appropriate locations in the adult stage, similar to the processes observed during early embryogenesis. Granule cells that differentiated from atoh1c-expressing neural progenitor cells in adulthood also contributed to cerebellar regeneration. Furthermore, inhibition of transforming growth factor-β (TGF-β) signaling, either via chemical inhibitors or CRISPR/Cas9, suppressed atoh1a/c expression and reduced granule cell numbers in larvae. Chemical inhibition of TGF-β signaling also suppressed neural progenitor cell proliferation, atoh1c expression, and granule cell neurogenesis in the adult cerebellum. These findings demonstrate that TGF-β signaling is essential for granule cell production from progenitor cells throughout the lifespan of zebrafish.