Pyramidal neurons in the deep layers of the cerebral cortex can be classified into two major classes: callosal projection neurons and long-range subcortical neurons. We and others have shown that a gene expressed specifically by subcortical projection neurons, Fezf2, is required for the formation of axonal projections to the spinal cord, tectum, and pons. Here, we report that Fezf2 regulates a decision between subcortical vs. callosal projection neuron fates. callosal ͉ cell fate ͉ zinc finger transcription factor ͉ corticospinal tract ͉ axon guidance

The Olf-1/EBF helix-loop-helix (HLH) transcription factor has been implicated in olfactory gene regulation and in B-cell development. Using homology screening methods, we identified two additional Olf-1/EBF-like cDNAs from a mouse embryonic cDNA library. The Olf-1/EBF-like (O/E) proteins O/E-1, O/E-2, and O/E-3 define a family of transcription factors that share structural similarities and biochemical activities. Although these O/E genes are expressed within olfactory epithelium in an identical pattern, they exhibit different patterns of expression in the developing nervous system. Although O/E-1 mRNA is present in several tissues in addition to olfactory neurons and developing B-cells, O/E-2 and O/E-3 are expressed at high levels only in olfactory tissue. In O/E-1 knock-out animals, the presence of two additional O/E family members in olfactory neurons may provide redundancy and allow normal olfactory neurodevelopment. Further, the identification of the O/E family of HLH transcription factors and their embryonic expression patterns suggest that the O/E proteins may have a more general function in neuronal development.

The mammalian Olf1/EBF (O/E) family of repeated helix-loop-helix (rHLH)transcription factors has been implicated in olfactory system gene regulation,nervous system development and B-cell differentiation. Ebf(O/E1) mutant animals showed defects in B-cell lineage and brain regions where it is the only O/E family member expressed, but the olfactory epithelium appeared unaffected and olfactory marker expression was grossly normal in these animals. In order to further study the mammalian O/E proteins,we disrupted O/E2 and O/E3 genes in mouse and placed tau-lacZ and tau-GFP reporter genes under the control of the respective endogenous O/E promoters. Mice mutant for each of these genes display reduced viability and other gene-specific phenotypes. Interestingly, both O/E2 and O/E3 knockout mice as well as O/E2/O/E3 double heterozygous animals share a common phenotype:olfactory neurons (ORN) fail to project to dorsal olfactory bulb. We suggest that a decreased dose of O/E protein may alter expression of O/E target genes and underlie the ORN projection defect.