Illumination of neural development by in vivo clonal analysis

Stem cell competition driven by the Axin2-p53 axis controls brain size during murine development

Cell competition acts as a quality-control mechanism that eliminates cells less fit than their neighbors to optimize organ development. Whether and how competitive interactions occur between neural progenitor cells (NPCs) in the developing brain remains unknown. Here, we show that endogenous cell competition occurs and intrinsically correlates with the Axin2 expression level during normal brain development. Induction of genetic mosaicism predisposes Axin2-deficient NPCs to behave as "losers" in mice and undergo apoptotic elimination, but homogeneous ablation of Axin2 does not promote cell death. Mechanistically, Axin2 suppresses the p53 signaling pathway at the post-transcriptional level to maintain cell fitness, and Axin2-deficient cell elimination requires p53-dependent signaling. Furthermore, mosaic Trp53 deletion confers a "winner" status to p53-deficient cells that outcompete their neighbors. Conditional loss of both Axin2 and Trp53 increases cortical area and thickness, suggesting that the Axin2-p53 axis may coordinate to survey cell fitness, regulate natural cell competition, and optimize brain size during neurodevelopment.

A Spacetime Odyssey of Neural Progenitors to Generate Neuronal Diversity

To understand how the nervous system develops from a small pool of progenitors during early embryonic development, it is fundamentally important to identify the diversity of neuronal subtypes, decode the origin of neuronal diversity, and uncover the principles governing neuronal specification across different regions. Recent single-cell analyses have systematically identified neuronal diversity at unprecedented scale and speed, leaving the deconstruction of spatiotemporal mechanisms for generating neuronal diversity an imperative and paramount challenge. In this review, we highlight three distinct strategies deployed by neural progenitors to produce diverse neuronal subtypes, including predetermined, stochastic, and cascade diversifying models, and elaborate how these strategies are implemented in distinct regions such as the neocortex, spinal cord, retina, and hypothalamus. Importantly, the identity of neural progenitors is defined by their spatial position and temporal patterning factors, and each type of progenitor cell gives rise to distinguishable cohorts of neuronal subtypes. Microenvironmental cues, spontaneous activity, and connectional pattern further reshape and diversify the fate of unspecialized neurons in particular regions. The illumination of how neuronal diversity is generated will pave the way for producing specific brain organoids to model human disease and desired neuronal subtypes for cell therapy, as well as understanding the organization of functional neural circuits and the evolution of the nervous system.

Cascade diversification directs generation of neuronal diversity in the hypothalamus

The hypothalamus contains an astounding heterogeneity of neurons that regulate endocrine, autonomic, and behavioral functions. However, its molecular developmental trajectory and origin of neuronal diversity remain unclear. Here, we profile the transcriptome of 43,261 cells derived from Rax⁺ hypothalamic neuroepithelium to map the developmental landscape of the mouse hypothalamus and trajectory of radial glial cells (RGCs), intermediate progenitor cells (IPCs), nascent neurons, and peptidergic neurons. We show that RGCs adopt a conserved strategy for multipotential differentiation but generate Ascl1⁺ and Neurog2⁺ IPCs. Ascl1⁺ IPCs differ from their telencephalic counterpart by displaying fate bifurcation, and postmitotic nascent neurons resolve into multiple peptidergic neuronal subtypes. Clonal analysis further demonstrates that single RGCs can produce multiple neuronal subtypes. Our study reveals that multiple cell types along the lineage hierarchy contribute to fate diversification of hypothalamic neurons in a stepwise fashion, suggesting a cascade diversification model that deconstructs the origin of neuronal diversity.

Do the Cytoplast and Nuclear Material of Germinal Vesicle Oocyte Support Developmental Competence Upon Reconstruction with Embryonic/Somatic Nucleus

Maturation conditions and oocytes quality have substantial roles on developmental competence of unreconstructed or reconstructed oocytes. Cloning has been reported successfully with low efficiency through embryonic or somatic nuclear transfer into enucleated metaphase II oocytes. It has been suggested that introducing embryonic or somatic nucleus to cytoplast at earlier stage might improve reprogramming of the introduced nucleus. In addition, the synchronization between the donor nucleus and recipient cytoplasts might effect on reprogramming and further embryonic development. Therefore, the question arises; does the cytoplast of germinal vesicle (GV) oocyte containing nuclear sap improve developmental competence upon reconstruction with embryonic/somatic nucleus compared with MII cytoplast. It has been indicated that GV material is essential for remodeling of sperm or embryonic or somatic nucleus in GV oocyte cytoplast and their further embryonic developmental competence. GV cytoplasts could be obtained through micromanipulation. Different micromanipulation techniques of immature oocytes at different stages were adapted in addition to introducing donor nuclei at G0/G1, S and G2/M phase, and enucleolation technique as well. Upon micromanipulation, it could obtain GV cytoplasts; cumulus-free and without GV material, cumulus-complexes and without GV material, cumulus-free and with GV material, and cumulus-complexes and with GV material in addition to enucleolated GV oocytes. Therefore, this short review will address briefly the importance of maturation conditions, cumulus cells, oocyte quality, the techniques of enucleation GV oocyte, the cell cycle stage of the introduced donor cell, or nucleus for oocyte maturation and embryo development.