Neural peer pressure: intercellular dynamics and emergent phenotypes in the mosaic Rett syndrome brain.

During female embryogenesis, each cell randomly inactivates either the maternal or paternal X chromosome, creating a natural mix, or mosaic, of cells with a different active X chromosome. In X-linked neurological disorders such as Rett syndrome, this results in a brain composed of two cell populations: one expressing the mutant allele and one expressing the wildtype allele. Rather than behaving as independent entities, these populations influence one another through the exchange of extracellular vesicles, free-floating biomolecules, and direct cell-cell contacts. The bidirectional communication between mutant and wildtype cells can give rise to emergent behaviors that cannot be explained by cell-autonomous mechanisms alone. Particularly, rather than a simple dichotomy of two discrete cell states, emerging evidence suggests that the mosaic Rett syndrome brain contains a spectrum of cells with intermediate phenotypes. Hence, shifting focus from isolated cellular effects to cell population-level dynamics will be crucial for advancing our understanding of mosaic conditions and developing effective therapeutic strategies.