Neuroepithelial organoid patterning is mediated by a neighborhood watch mechanism

The role of microheterogeneity in cell fate decisions in neural progenitors and neural crest

Neuroprogenitors must integrate a multitude of signals, including gradients of morphogens, transcriptional programs, and temporal cues to generate an astonishing diversity of cell types inhabiting the nervous system. How do these different layers of information come together to influence cell fate in progenitor cells in a coordinated way? Here we provide a nuanced perspective on cell fate selection in the nervous system and neural crest lineage, suggesting that it is not a straightforward, deterministic process governed by rigid on-off switches. Instead, the process involves probabilistic transitions influenced by small variations - termed "microheterogeneity" - within a progenitor cell population. These minuscule differences between individual neural progenitor cells can result in significantly different outcomes, making certain fates more probable for some cells than others. Here we discuss the diversity of such examples and the theory behind, also providing future perspectives.

Small molecule valproic acid enhances ventral patterning of human neural tube organoids by regulating Wnt and Shh signalling

Valproic acid (VPA), a clinically approved small molecule, has been reported to activate Wnt signalling that is critical for dorsal-ventral (DV) patterning of neural tube. However, little is known about the impact of VPA on DV patterning process. Here, we show that even though VPA has a negative impact on the early formation of human neural tube organoids (hNTOs), it significantly enhances the efficiency of ventrally patterned hNTOs, when VPA is added during the entire differentiation process. RNA sequencing and RT-qPCR analysis demonstrates VPA activates endogenous Wnt signalling in hNTOs. Surprisingly, transcriptome analysis also identifies upregulation of genes for degradation of GLI2 and GLI3 proteins, whose truncated fragment are transcriptional repressors of Shh signalling. The Western-blot analysis confirms the increase of GLI3R proteins after VPA treatment. Thus, VPA might enhance ventral patterning of hNTOs through both activating Wnt, which can antagonise Shh signalling by inducing GLI3 expression, and/or inhibiting Shh signalling by inducing GLI protein degradation. We further obtain results to show that VPA still increases patterning efficiency of hNTOs with a weak influence on their early formation when the initiation time of VPA is delayed and its duration is reduced. Taken together, this study demonstrates that VPA enhances the generation of more reproducible hNTOs with ventral patterning, opening the avenues for the applications of hNTOs in developmental biology and regenerative medicine.

Mechanical Actuation of Organoids in Synthetic Microenvironments

Organoids are a powerful model system to explore the role of mechanical forces in sculpting emergent tissue cytoarchitecture. The modulation of the mechanical microenvironment is most readily performed using synthetic extracellular matrices (ECM); however, such materials provide passive, rather than active force modulation. Actuation technologies enable the active tuning of mechanical forces in both time and magnitude. Using such instruments, our group has shown that extrinsically imposed stretching on human neural tube organoids (hNTOs) enhanced patterning of the floor plate domain. Here, we provide a detailed protocol on the implementation of mechanical actuation of organoids embedded in synthetic 3D microenvironments, with additional details on methods to characterize organoid fate and behavior. Our protocol is easy to reproduce and is expected to be broadly applicable to investigate the role of active mechanics with in vitro model systems.