Modulation of Nudt21 levels reveals dose-dependent roles of alternative polyadenylation in tissue regeneration.

Stem cells continually self-renew and differentiate to sustain tissue homeostasis, yet the role of post-transcriptional mechanisms in guiding these processes remains incompletely understood. Here, we demonstrate that the regulation of 3'UTR length via alternative mRNA polyadenylation (APA) is essential for stem cell function across diverse tissues. Modulating the APA regulator Nudt21 reveals that stem cell self-renewal and differentiation depend on distinct dosage thresholds and thus can be uncoupled. Specifically, moderate Nudt21 suppression  elicits a maturation arrest of stem cells due to 3'UTR-shortening of differentiation-associated mRNAs that escape miRNA regulation and perturb ceRNA networks. By contrast, complete Nudt21 suppression additionally shortens the 3'UTRs of mRNAs encoding essential multiprotein complexes, including the nuclear pore, leading to complex destabilization, proteotoxic stress, DNA damage, and cell cycle arrest. Critically, deletion of the alternative 3'UTRs of individual nucleoporins recapitulates defects observed with Nudt21 loss. We further demonstrate that the co-translational assembly of dozens of protein complexes is impaired in Nudt21-deficient cells, providing a mechanistic framework for compromised complex integrity. Collectively, our results show that APA plays distinct, dose-dependent roles in stem cell homeostasis by fine-tuning the expression of differentiation-associated genes and coordinating the biogenesis of multiprotein complexes essential for cell cycle progression.