Aims Muscleblind‐like 2 (MBNL2) plays a crucial role in regulating alternative splicing during development and mouse loss of MBNL2 recapitulates brain phenotypes in myotonic dystrophy (DM). However, the mechanisms underlying DM neuropathogenesis during brain development remain unclear. In this study, we aim to investigate the impact of MBNL2 elimination on neuronal development by Mbnl2 conditional knockout (CKO) mouse models. Methods To create Mbnl2 knockout neurons, cDNA encoding Cre‐recombinase was delivered into neural progenitors of Mbnl2flox/flox mouse brains by in utero electroporation. The morphologies and dynamics of dendritic spines were monitored by confocal and two‐photon microscopy in brain slices and live animals from the neonatal period into adulthood. To investigate the underlying molecular mechanism, we further detected the changes in the splicing and molecular interactions of proteins associated with spinogenesis. Results We found that Mbnl2 knockout in cortical neurons decreased dendritic spine density and dynamics in adolescent mice. Mbnl2 ablation caused the adducin 1 (ADD1) isoform to switch from adult to fetal with a frameshift, and the truncated ADD1 failed to interact with alpha‐II spectrin (SPTAN1), a critical protein for spinogenesis. In addition, expression of ADD1 adult isoform compensated for the reduced dendritic spine density in cortical neurons deprived of MBNL2. Conclusion MBNL2 plays a critical role in maintaining the dynamics and homeostasis of dendritic spines in the developing brain. Mis‐splicing of downstream ADD1 may account for the alterations and contribute to the DM brain pathogenesis.

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