# 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

## TL;DR

This study shows that TGF-β signaling is crucial for the generation of cerebellar granule cells from atoh1c-expressing progenitors in zebrafish, both during development and in adult regeneration.

## Contribution

The study reveals the role of TGF-β signaling in regulating atoh1c-expressing progenitors for granule cell production in zebrafish across life stages.

## Key findings

- Granule cells in adult zebrafish cerebellum differentiate from atoh1c-expressing progenitors and contribute to regeneration.
- TGF-β signaling inhibition reduces atoh1a/c expression and granule cell numbers in larvae and adults.
- Proliferating progenitors in the molecular layer generate granule cells that migrate to the granular layer.

## Abstract

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.

Proliferating neural progenitor cells that generate granule cells are located in the molecular layer (ML) of the cerebellum. Among these, some atoh1c‐expressing cells begin to differentiate and migrate to the granular layer (GL), where they localize as mature granule cells. A similar process occurs in the valvula cerebelli (Va), the anterior part of the cerebellum. In the torus longitudinalis (TL) of the mesencephalon, granule cells are derived from atoh1c‐expressing cells in larvae, but their production is limited in adults.