SMAD3 and Parkinson disease: Disruption of this cascade manifests in disease‐specific mechanisms: promoting Aβ production via BACE1/γ‐secretase in Alzheimer's; impairing α‐synuclein clearance in Parkinson's; disrupting GLUT4 translocation and insulin signaling in diabetes; exacerbating oxidative damage and mitochondrial dysfunction in cardiovascular and neuronal injury; and accelerating fibrosis and sustained inflammation in renal and pulmonary diseases via NLRP3 and TGF‐β/Smad3 signaling.