The amyloid precursor protein (APP) is widely studied as the source of amyloid-beta in Alzheimer disease, but its broader functions in neurodevelopment remain incompletely defined. In the context of Trisomy 21 (Down syndrome), where APP is triplicated, the impact of APP dosage and structural variants on human neurogenesis is of particular interest. The authors used trisomic human induced pluripotent stem cells (iPSCs) to probe APP-specific contributions to two distinct stages of neurogenesis: terminal differentiation of neural progenitor cells (NPCs) into post-mitotic neurons, and neuronal structural maturation as reflected by neurite outgrowth.
To isolate APP-specific effects in trisomic cells, the study produced a panel of isogenic trisomy 21 iPSC lines varying only in APP status. This collection included lines with reduced APP dosage (monosomic APP), a complete APP knockout (null), and lines carrying targeted mutations in the APP extracellular domain. These isogenic variants enabled direct comparisons that control for background genomic differences inherent to patient-derived lines.
The authors assayed two distinct neurogenic outcomes in the panel of APP variant lines. First, they evaluated terminal differentiation of human NPCs into post-mitotic neurons to measure the ability of progenitors to exit the cell cycle and adopt neuronal fates. Second, they examined extended neuron structure, specifically neurite outgrowth, as an index of neuronal maturation and structural development. Studying these two steps allowed interrogation of whether APP impacts progenitor fate decisions, neuronal morphology, or both.
Complete loss of APP produced pronounced defects across both neurogenic steps studied. APP-null trisomic lines exhibited reduced terminal differentiation of NPCs into post-mitotic neurons, indicating that APP is required for normal progression from progenitor to neuron in this model. In addition, absence of APP diminished proper neurite development and the extended structural maturation of neurons. These observations support a requirement for APP in both progenitor differentiation and subsequent neuronal structural maturation.
Further analyses of the APP-null and extracellular domain mutant lines revealed that the two observed neurogenic defects likely result from different mechanisms. The deficit in terminal differentiation correlated with lowered APP dosage: either complete absence or reduced (monosomic) APP prolonged NPC cell cycling, impeding timely exit from the cell cycle and neuronal differentiation. By contrast, defects in neurite extension were linked to mutations in the APP extracellular domain, suggesting that structural integrity of APP — rather than simple dosage — is critical for the protein’s role in neuron morphogenesis.
Because NPC cell-cycle dynamics and terminal differentiation are regulated by Notch signaling — and because both APP and Notch are substrates for gamma-secretase cleavage — the authors tested whether APP dosage effects might act indirectly via changes in Notch activity. They treated NPCs with Compound E, a gamma-secretase inhibitor that suppresses Notch signaling. Treatment with Compound E restored neurogenesis levels in APP-depleted lines, supporting the hypothesis that reduced APP dosage influences progenitor fate indirectly through gamma-secretase–dependent modulation of Notch signaling. In contrast, neurite outgrowth impairments associated with extracellular domain disruption were not attributed to this indirect pathway, consistent with a more direct, structural role for APP in neuronal maturation.
This work provides a characterized resource of APP mutant isogenic trisomy 21 iPSC lines that can be used to dissect APP-specific contributions to human neural development. The findings highlight a dynamic interplay between APP dosage and Notch/gamma-secretase signaling in determining NPC cycling and terminal differentiation, while implicating the APP extracellular domain in direct structural roles for neurite extension. Together, these results raise further questions about how APP copy number and protein structure orchestrate progenitor fate decisions and neuronal maturation during human brain development in the context of Trisomy 21.
The article is presented as a preprint and has not undergone peer review. Specific experimental details, quantitative values, and extended data are reported in the full preprint and supplementary materials; those details were not exhaustively repeated here and should be consulted in the source. The authors declared no competing interests and listed NIH and other funding sources in the original report.