The CSF contained Wnt signaling activity (Zhou et al., 2006), based Ixazomib cost upon phosphorylation of LRP6, a Wnt coreceptor in response to CSF exposure (Figure 7A). Several Wnt ligands were expressed along the ventricular surface and in the choroid plexus (Figure 7B and data not shown; Grove et al., 1998). Frizzled (Fz) receptors, which bind LRP6 to transduce Wnt signals, showed enhanced expression in ventricular progenitors (Figure 7B and data not shown; Zhou et al., 2006), suggesting that CSF may distribute Wnts to precursors throughout the ventricular surface. Additional

signaling activities that influence cortical development were also found in the CSF, with responsive cells seen broadly in the ventricular zone. There were dynamic levels of bone morphogenetic protein (Bmp) activity in the CSF during different stages of cortical development (Figure 7C). Using a luciferase-based

assay in which overall Bmp activity can be quantified between 0.1 and 100 ng/ml (data not shown), we found that Bmp activity in the CSF decreased during embryogenesis and peaked in adulthood (Figure 7C). CSF-borne Bmp activity may be responsible for stimulating progenitors widely throughout the cortical ventricular zone in vivo, based on widespread labeling for nuclear phospho-SMAD1/5/8 (Figure 7D) in the absence of any known Bmp ligands localizing to the ventricular zone (Shimogori et al., 2004), whereas Bmps 2, 4, 5, and 7 are expressed in embryonic and adult choroid plexus (Figure 7E; Hébert et al., 2002 and Shimogori et al., 2004). Moreover, growth and differentiation factors RAD001 nmr 3 and 8 (GDF3 and GDF8), both members of the TGF-β superfamily of proteins that can influence Bmp signaling (Levine and Brivanlou, 2006) were found in our MS analyses of CSF (data not very shown),

though we do not consider our MS analysis to have recovered all potential smaller ligands in the CSF. Retinoic acid (RA) (Haskell and LaMantia, 2005 and Siegenthaler et al., 2009) activity in CSF also varied over the course of cortical development (Figure 7F). A luciferase-based assay that quantifies RA activity ranging between 10−9 and 10−6M (data not shown) revealed that RA activity in CSF peaked early and decreased in adulthood (Figure 7F). In parallel, RA responsive cortical progenitors localized to the developing ventricular zone (Figure 7G). Similar to Wnts and Bmps, RA is most likely released into CSF since RA synthetic and catabolic enzymes were expressed in the choroid plexus (Figure 7H) and meninges (data not shown). Thus, CSF shows bioavailability of a wide range of activities known to regulate neurogenesis, patterning, and neuronal survival in the cerebral cortex and throughout the CNS. We show that the CSF plays an essential, active role in distributing signals in the central nervous system.