The transforming growth factor- (TGF-) signaling pathway serves critical functions in
The transforming growth factor- (TGF-) 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. aspects of brain development, and a growing literature suggests that they fulfill important functions in the adult brain as well1. The 1alpha, 25-Dihydroxy VD2-D6 founding members of this family, TGF-1, 2 and 3, are dimeric polypeptide growth factors2 which are broadly expressed in the brain3. Canonical TGF- signaling is initiated by ligand binding to a high-affinity transmembrane TGF- type II receptor (TRII), which subsequently phosphorylates TGF- type I receptor activin-like kinase 5 (TRI or ALK5)4. This leads to phosphorylation of Smad2 and Smad3 proteins, which form a heteromeric complex with Smad4 and translocate into the nucleus where they regulate transcription4. TGF-s can also activate other signaling cascades in a context-dependent manner, such as MAPK, JNK, and PKC pathways5. TGF- type I receptor ALK5 is highly expressed in migrating neurons of the developing cortex6 and TGF- signaling regulates self-renewal of neural stem cells in the developing midbrain7. TGF-s have also been shown to promote the sprouting and elongation of neurites in dissociated hippocampal cultures8 and to regulate synaptic growth in depending on TGF- type I receptor9. 1alpha, 25-Dihydroxy VD2-D6 Moreover, TGF- signaling was reported to mediate axon specification during brain development10. In the adult brain TGF-s seem to have broad neuroprotective functions11. They are induced in response to injury and have thus been implicated in neurodegenerative diseases12. For example, deficiency in TGF-1 results in synapto-dendritic degeneration and increased susceptibility to excitotoxic injury13, and reduced expression of TRII in neurons promotes neurodegeneration in a mouse model of Alzheimer’s disease14. Consistent with its function in regulating developmental neurogenesis, TGF-1 can reduce adult neurogenesis by inhibiting cell cycle progression in neural progenitor cells and promoting 1alpha, 25-Dihydroxy VD2-D6 stem cell quiescence15, 16. Adult neurogenesis persists in the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus; the latter process exerts an important role in hippocampus-dependent learning, memory, and other cognitive functions17. Neurogenesis in the adult brain is regulated through a number of signaling pathways18 and in response to physiological stimuli such as aging, exercise, and CNS injury19. Many of these 1alpha, 25-Dihydroxy VD2-D6 factors regulate early events of neurogenesis, including quiescence, proliferation, and fate specification of neural stem cells20 but relatively little is known about factors that regulate the subsequent survival, maturation, and functional integration of newborn neurons. Here we demonstrate that TGF- signaling serves a critical role in late stage adult neurogenesis. We observed that Smad2/3-dependent signaling is prominently activated in dentate gyrus postmitotic immature neurons and adult mature neurons but not in radial glia-like stem cells or neural progenitor cells. Genetic knockdown of TGF- type I receptor ALK5 in proliferating progenitors in the dentate gyrus resulted in reduced survival, migration, and shorter dendrite length Rabbit Polyclonal to CSGALNACT2 of newborn neurons, while activation of this receptor in transgenic mice had the opposite 1alpha, 25-Dihydroxy VD2-D6 effects and improved hippocampus-dependent working and spatial memory. Our findings demonstrate that TGF- signaling through ALK5 is necessary and sufficient to maintain late events during adult hippocampal neurogenesis. Results Canonical TGF- signaling is active in the dentate gyrus We had reported earlier that within the mouse brain TGF- signaling is highest in the hippocampus21. To explore this further, we dissected brains of previously described unmanipulated Smad binding elements (SBE)-luciferase reporter mice22 into different brain regions. In these mice, luciferase is expressed under the SBE promoter and its activity is positively correlated with TGF- signaling. We found highest luciferase activity in the adult dentate gyrus, lower signals in the (CA) area of the hippocampus, and no signal in the cerebellum or in non-transgenic littermate control mice (Fig. 1a). Immunohistochemical staining of the adult dentate gyrus showed that under physiological conditions, p-Smad2, downstream of TGF- signaling was prominently expressed in the granule zone of the dentate gyrus (Fig. 1b). More than 95% of p-Smad2+ cells expressed NeuN (mature neuron marker) (Fig. 1b,c). In contrast, few Sox2+GFAP+ radial glia-like cells, MCM2+ or Tbr2+ neural progenitor cells in the dentate gyrus showed detectable p-Smad2 immunoreactivity (Fig. 1b,c). Interestingly, almost 5% of p-Smad2+ cells expressed doublecortin (DCX, neuroblast and immature neuron marker) (Fig. 1b,c). DCX expressing cells are highly heterogeneous and can be divided into proliferating neuroblasts and postmitotic immature neurons according to their proliferative activity23. By using proliferating cell nuclear antigen (PCNA) as a.