Modeling Tauopathy-associated neurodegeneration in human iPSC-derived 2D and 3D retinal cultures

Tau is a crucial protein that plays a significant role in maintaining neuronal structure and function. During retinogenesis, tau is involved in the development and maturation of retinal tissue. Previous studies have identified pathological hallmarks of tauopathies, such as Frontotemporal dementia (FTD), in the retina. However, the precise molecular mechanisms and the direct sequence of events underlying this process remain poorly understood. In this study, we investigated the changes associated with tau during retinogenesis using a human induced pluripotent stem cell (iPSC) cell line harboring an intronic IVS10+16 tau mutation, associated with to FTD. By utilizing both 2D and 3D retinal models derived from tau-mutant iPSCs and their isogenic controls, we examined the spatiotemporal development of retinal progenitor cells and post-mitotic neurons, considering the role of tau in regulating the growth and differentiation of retinal neurons. Through gene expression and immunofluorescence analysis, we observed that mutant cultures exibited delayed neurogenesis during retinal development, which disrupted the balance between the proliferation and differentiation of neuronal progenitor cells. Furthermore, we confirmed an early and increased expression of 4R tau isoforms in the mutant cultures, leading to an altered 4R/3R isoform ration. Since tau is known to promote neuronal survival and prevent apoptosis, we also monitored the pathological hallmarks of tau throughout the differentiation process, and we identified phosphorylated site during the early stage of IVS10+16 retinal differentiation. Additionally, we investigated the formation of functional neural circuits and observed that the IVS10+16 tau mutation deeply affects synaptic maturation, influencing the organization of pre and postsynaptic components. Moreover, functional analysis of Ca2+ oscillations in 2D neuronal cultures showed that the IVS10+16 tau mutation impaired neuronal activity reducing both the frequency and synchronicity of Ca2+ oscillations. These results suggest that the inherited intronic tau mutation has a significant impact on neuronal maturation during retinal development. We can speculate that that defects in neurodevelopment, as seen in the presence of inherited FTD associated tau mutation, may initially be compensated for by homeostatic mechanisms. However, over time, the loss of these compensatory mechanisms can ultimately lead to neurodegeneration.