Single cell transcriptomic analyses reveal the impact of bHLH factors on human retinal organoid development

The developing retina expresses multiple bHLH transcription
factors. Their precise functions and interactions in uncommitted
retinal progenitors remain to be fully elucidated. Here, we
investigate the roles of bHLH factors ATOH7 and Neurog2 in human ES
cell-derived retinal organoids.  Single-cell transcriptome
analyses identify three states of proliferating retinal progenitors:
pre-neurogenic, neurogenic, and cell cycle-exiting
progenitors. Each shows different expression profile of bHLH
factors. The cell cycle-exiting progenitors feed into a
postmitotic heterozygous neuroblast pool that gives rise to early born
neuronal lineages. Elevating ATOH7 or Neurog2 expression
accelerates the transition from the pre-neurogenic to the neurogenic
state, and expands the exiting progenitor and neuroblast
populations. In addition, ATOH7 and Neurog2 significantly, yet
differentially, enhance retinal ganglion cell and cone photoreceptor
production. Moreover, single-cell transcriptome analyses reveal
that ATOH7 and Neurog2 assert positive autoregulation, suppress key bHLH
factors associated with the neurogenic progenitors, and elevate bHLH
factors expressed by exiting progenitors and differentiating
neuroblasts. This study thus provides novel insight regarding how
ATOH7 and Neurog2 impact human retinal progenitor behaviors and neuroblast
fate choices.