Alzheimer's disease (AD) is a neurodegenerative disorder characterized by deregulation of neuronal cell cycle and differentiation control eventually resulting in cell death. During brain development, neuronal differentiation is regulated by Smad proteins, which are elements of the canonical transforming growth factor β (TGF-β) signaling pathway, linking receptor activation to gene expression. In the normal adult brain, Smad proteins are constitutively phosphorylated and predominantly localized in neuronal nuclei. Under neurodegenerative conditions such as AD, the subcellular localization of their phosphorylated forms is heavily disturbed, raising the question of whether a nuclear Smad deficiency in neurons might contribute to a loss of neuronal differentiation control and subsequent cell cycle re-entry. Here, we show by luciferase reporter assays, electromobility shift, and RNA interference (RNAi) technique a direct binding of Smad proteins to the CDK4 promoter inducing transcriptional inhibition of cell cycle-dependent kinase 4 (Cdk4). Mimicking the neuronal deficiency of Smad proteins observed in AD in cell culture by RNAi results in elevation of Cdk4 and retardation of neurite outgrowth. The results identify Smad proteins as direct transcriptional regulators of Cdk4 and add further evidence to a Smad-dependent deregulation of Cdk4 in AD, giving rise to neuronal dedifferentiation and cell death.