GAS1 is required for NOTCH-dependent facilitation of SHH signaling in the ventral forebrain neuroepithelium

Hedgehog-dependent and hedgehog-independent roles for growth arrest specific 1 in mammalian kidney morphogenesis

Growth arrest specific 1 (GAS1) is a key regulator of mammalian embryogenesis, best known for its role in hedgehog (HH) signaling, but with additional described roles in the FGF, RET, and NOTCH pathways. Previous work indicated a later role for GAS1 in kidney development through FGF pathway modulation. Here, we demonstrate that GAS1 is essential for both mesonephrogenesis and metanephrogenesis - most notably, Gas1 deletion in mice results in renal agenesis in a genetic background-dependent fashion. Mechanistically, GAS1 promotes mesonephrogenesis in a HH-dependent fashion, performing a unique co-receptor function, while promoting metanephrogenesis in a HH-independent fashion, acting as a putative secreted RET co-receptor. Our data indicate that Gas1 deletion leads to renal agenesis through a transient reduction in metanephric mesenchyme proliferation - a phenotype that can be rescued by exogenous RET pathway stimulation. Overall, this study indicates that GAS1 contributes to early kidney development through the integration of multiple different signaling pathways.

Toll-like receptors as a missing link in Notch signaling cascade during neurodevelopment

Neurodevelopment encompasses a complex series of molecular events occuring at defined time points distinguishable by the specific genetic readout and active protein machinery. Due to immense intricacy of intertwined molecular pathways, extracting and describing all the components of a single pathway is a demanding task. In other words, there is always a risk of leaving potential transient molecular partners unnoticed while investigating signaling cascades with core functions-and the very neglected ones could be the turning point in understanding the context and regulation of the signaling events. For example, signaling pathways of Notch and Toll-like receptors (TLRs) have been so far unrelated in the vast body of knowledge about neurodevelopment, however evidence from available literature points to their remarkable overlap in influence on identical molecular processes and reveals their potential functional links. Based on data demonstrating Notch and TLR structural engagement and functions during neurodevelopment, along with our description of novel molecular binding models, here we hypothesize that TLR proteins act as likely crucial components in the Notch signaling cascade. We advocate for the hypothesized role of TLRs in Notch signaling by: elaborating components and features of their pathways; reviewing their effects on fates of neural progenitor cells during neurodevelopment; proposing molecular and functional aspects of the hypothesis, along with venues for testing it. Finally, we discuss substantial indications of environmental influence on the proposed Notch-TLR system and its impact on neurodevelopmental outcomes.

SHH and Notch regulate SOX9+ progenitors to govern arcuate POMC neurogenesis

Pro-opiomelanocortin (POMC)-expressing neurons in the hypothalamic arcuate nucleus (ARC) play key roles in feeding and energy homoeostasis, hence their development is of great research interest. As the process of neurogenesis is accompanied by changes in adhesion, polarity, and migration that resemble aspects of epithelial-to-mesenchymal transitions (EMTs), we have characterised the expression and regulation within the prospective ARC of transcription factors with context-dependent abilities to regulate aspects of EMT. Informed by pseudotime meta-analysis of recent scRNA-seq data, we use immunohistochemistry and multiplex in situ hybridisation to show that SOX2, SRY-Box transcription factor 9 (SOX9), PROX1, Islet1 (ISL1), and SOX11 are sequentially expressed over the course of POMC neurogenesis in the embryonic chick. Through pharmacological studies ex vivo, we demonstrate that while inhibiting either sonic hedgehog (SHH) or Notch signalling reduces the number of SOX9+ neural progenitor cells, these treatments lead, respectively, to lesser and greater numbers of differentiating ISL1+/POMC+ neurons. These results are consistent with a model in which SHH promotes the formation of SOX9+ progenitors, and Notch acts to limit their differentiation. Both pathways are also required to maintain normal levels of proliferation and to suppress apoptosis. Together our findings demonstrate that hypothalamic neurogenesis is accompanied by dynamic expression of transcription factors (TFs) that mediate EMTs, and that SHH and Notch signalling converge to regulate hypothalamic cellular homoeostasis.