ALK5-dependent TGF-β signaling is a major determinant of late stage adult neurogenesis

The transforming growth factor-β (TGF-β) signaling pathway serves critical functions in central nervous system (CNS) development, but apart from its proposed neuroprotective actions, its physiological role in the adult brain is unclear. We observed a prominent activation of TGF-β signaling in the adult dentate gyrus and expression of downstream Smad proteins in this neurogenic zone. Consistent with a function of TGF-β signaling in adult neurogenesis, genetic deletion of the TGF-β receptor ALK5 reduced the number, migration, and dendritic arborization of newborn neurons. Conversely, constitutive activation of neuronal ALK5 in forebrain caused a striking increase in these aspects of neurogenesis and was associated with higher expression of c-fos in newborn neurons and with stronger memory function. Our findings describe a new and unexpected role for ALK5-dependent TGF-β signaling as a regulator of the late stages of adult hippocampal neurogenesis which may have implications for changes in neurogenesis during aging and disease.