Expression of the secreted factors noggin and bone morphogenetic proteins in the subependymal layer and olfactory bulb of the adult mouse brain.

The antagonism between noggin and the bone morphogenetic proteins (BMPs) plays a key role during CNS morphogenesis and differentiation. Recent studies indicate that these secreted factors are also widely expressed in the postnatal and adult mammalian brain in areas characterized by different types of neural plasticity. In particular, significant levels of noggin and BMP expression have been described in the rodent olfactory system. In the mammalian forebrain, the olfactory bulb (OB) and associated subependymal layer (SEL) are documented as sites of adult neurogenesis. Here, using multiple approaches, including the analysis of noggin-LacZ heterozygous mice, we report the expression of noggin and two members of the BMP family, BMP4 and BMP7, in these regions of the adult mammalian forebrain. We observe that along the full extent of the SEL, from the lateral ventricle to the olfactory bulb, noggin and BMP4 and 7 are mainly associated with the astrocytic glial compartment. In the OB, BMP4 and 7 proteins remain primarily associated with the SEL while strong noggin expression was also found in cells located in different OB layers (i.e. granule, external plexiform, glomerular layers). Taken together our data lead us to hypothesize that within the SEL the antagonism between noggin and BMPs, both produced by the glial tubes, act through autocrine/paracrine inductive mechanisms to maintain a neurogenetic environment all the way from the lateral ventricle to the olfactory bulb. In the OB, their expression patterns suggest multiple regulatory roles on the unusual neural plasticity exhibited by this region.