Neuronal Cell-Cycle Re-entry Defines Divergent Outcomes Through Replication-Dependent DNA Damage in ALS.

Cell-cycle dysregulation has emerged as a shared mechanism of neuronal loss across neurodegenerative diseases (NDDs), including amyotrophic lateral sclerosis (ALS), Alzheimer's disease, and Parkinson's disease. In post-mitotic neurons, aberrant reactivation of cell-cycle signaling precedes degeneration, yet the upstream triggers and functional consequences of this process remain poorly defined. Nucleocytoplasmic transport (NCT) dysfunction, a hallmark of ALS and related disorders, disrupts the spatial distribution of key regulatory proteins and may contribute to maladaptive cell-cycle activation. Our recent evidence suggests that impaired nuclear import may initiate, rather than merely accompany, neuronal cell-cycle re-entry. Here, we show that cell-cycle activation in motor neurons distinguishes molecular subtypes and outcomes in ALS. We analyzed the AnswerALS transcriptomic cohort and identified a patient cluster characterized by robust upregulation of cyclins B and D. Clusters with lower levels of cell-cycle gene expression exhibited accelerated ALSFRS-R decline, whereas the highest cyclin-expressing cluster demonstrated comparatively improved functional trajectories over time. To test whether NCT disruption can mechanistically drive aberrant cell-cycle activation, we pharmacologically inhibited importin-β in human iPSC-derived spinal motor neurons. NCT disruption induced widespread proteomic mislocalization, including TDP-43 pathology, and triggered a transient wave of cell-cycle activity preceding neuronal death. Mechanistically, we identified DNA-replication initiation as a pathological event driving degeneration and demonstrated that selective inhibition of G1/S-associated CDK4/6 activity confers neuroprotection. Together, these findings link impaired nuclear import to maladaptive cell-cycle reactivation in neurons and highlight stage-specific engagement of the cell-cycle machinery as a determinant of neuronal vulnerability in ALS.