Developmental wave of programmed ganglion cell death in human retinal organoids

Abstract

The delicate and complex structure of the neural retina that enables proper visual function is achieved during embryonic development through a precise balance of proliferation, differentiation, and cell death. Retinal ganglion cells (RGC), the only output neurons of the retina, show a steady increase in numbers during development except for two conserved waves of developmental cell death. However, the mechanisms responsible for these phenomena in the human retina are incompletely understood. In this work, we took advantage of human induced pluripotent stem cell (hiPSC)-derived retinal organoids to explore these questions. Using different markers and quantitative techniques in three different hiPSC lines, we found a consistent decrease in RGC numbers at week 8 of differentiation, corresponding to the timing of the early developmental wave described in other vertebrates. This decrease coincided with a peak in caspase 3 activation and TUNEL(+) staining, suggesting an apoptotic mechanism. Notably, this occurred without caspase 9 activation or an increased BAX/BCL2 ratio, but with elevated caspase 8 activation, indicating involvement of the extrinsic apoptotic pathway. Together, these results show for the first time the intrinsic ability of the human retina to regulate RGC numbers through programmed cell death, providing insight into conserved developmental mechanisms and informing the use of retinal organoids in basic and translational research.