Silva, T.;Iijima, E.;Ampartzidis, I.;Cooksley, G.;Paya, B.;Ucuncu, E.;Holder, D.;Smith, I.;Smith, J.;Giobbe, G.;Coppi, P.;Millen, K.;Clarke, J.;Livesey, F.;Greene, N.;Haldipur, P.;Galea, G.;Alexandre, P.

Down syndrome (DS) is associated with altered brain development, especially in the cerebellum. However, how trisomy 21 (TS21) is linked to cerebellar defects remains poorly understood. Cerebellar organoids reveal that TS21 leads to extracellular matrix (ECM) alterations, perturbing downstream pathways linked to reduction of apical RAB11 + endosomes. These changes lead to impaired apical maintenance and altered progenitor composition. Significantly, ECM-enriched culture rescues apical defects in TS21 organoids, supporting a functional role for ECM in preserving apical organisation and progenitor niche integrity. In the developing human DS cerebellum and 2D neural cultures, where exogenous ECM is present, severe epithelial disorganisation is attenuated. Nevertheless, ECM organisation and mechanical properties remain altered in 2D cultures. Our findings identify TS21-driven ECM abnormalities as a mechanism impairing apical integrity, progenitor composition and mechanical proprieties in neural stem cell models of DS. Altogether, we established apical maintenance instability as an early developmental defect contributing for DS neuropathology.