1.7.1 Fezl role in Xenopus and zebrafish
The Fez gene was first identified and cloned as a novel transcription factor containing six C2H2 type zinc finger domains highly conserved among Drosophila, zebrafish, mouse and human (Matsuo-Takasaki, M., et al., 2000). It has been shown that Fez mRNA is expressed during mid to late-gastrulation at the rostro-medial regions of the anterior neural plate in Xenopus embryos and localizes within the prospective telencephalic region while neurulation progresses. A subsequent study in zebrafish identified a similar to fez protein (95,7% identity within the zinc finger regions), which was named fez-like (Fezl).
The fezl transcript was detected in the anterior edge of the neuroectoderm, the prospective dorsal forebrain, as well as the ventral forebrain overlying the prechordal plate from the late gastrula to mid-segmentation stage during zebrafish development (Hashimoto, H. et al., 2000). Interestingly, Fezl expression was elevated in embryos where Wnt signaling was inhibited and reduced in the ones expressing Wnt8b, indicating a role in the induction of anterior neuroectoderm. This role was further reinforced by the fact that overexpression of fez in zebrafish embryos induces ectopic expression of dlx2 and dlx6, two genes involved in brain development, whereas its morpholino-based knockdown inhibits dlx2 expression in the ventral forebrain (Yang, Z. et al., 2001). Additionally, Fezl gain of function embryos exhibit a selective downregulation of Wnt1 and Pax2a while its misexpression leads to the expansion of the telencephalon and hypothalamus at the expense of other forebrain and midbrain regions (Jeong, J-Y. et al., 2007).
Furthermore, the zebrafish mutant too few (tof) in which the fezl gene is disrupted displays deficits in the development of diencephalic monoaminergic neurons in a non-cell autonomous way (Levkowitz, G. et al., 2002). This is most likely accomplished by inducing the expression of neogenin1 with which tof is coexpressed in DA progenitor domains of the basal forebrain (Jeong, J-Y. et al., 2006). Fezl control on hypothalamic differentiation is also exerted by its regulation of the homeodomain protein Orthopedia (Opt) at two distinct hypothalamic nuclei that produce isotocin expressing (IT) and DA neurons (Blechman, J. et al., 2007). Except for embryonic development in zebrafish, fezl is also expressed by radial glial progenitor cells of the adult telencephalic ventricular zone as well as by postmitotic neurons during adult neurogenesis (Berberoglu, M.A. et al., 2009).
1.7.2 Fezl role in mouse
In mouse embryos the fezl transcript can be detected at E11 in both rostral telencephalic hemispheres with high expression in the cortex and hippocampus and fainter expression in the anterior hypothalamus and the preoptic area (Matsuo-Takasaki, M., et al., 2000). The function of Fezl in mouse was revealed after the generation of Fezl mutant mice (Hirata, T. et al., 2004). This study showed that Fezl expression is apparent in the mouse prospective forebrain as early as E8.5. At E13.5 Fezl is detected in the nasal septum, thalamic eminence, ventral thalamus and
hypothalamus and dorsal telencephalon while by E15.5 it can be found in the cortical plate, striatum, hypothalamus and mamillary body. As development progresses its expression in the cortex is restricted to deep layers, mostly subcerebral projection neurons of layer V and to a much lesser extent in layer VI (Molyneaux, BJ. et al., normally resulting in the subsequent reduction and abnormal development of thalamocortical axons (TCA) (Komuta, Y. et al., 2007).
Fig. 8. Expression pattern of Fezl mRNA at different developmental stages.A. Enrichment of Fezl in CSMN neurons. B and C. Fezl expression in the cortex at E13.5 shows a high rostrolateral- low caudomedial pattern. D. As development progresses expression gets restricted mainly to layer V and less to layer VI neurons. E. Retrograde labeling with DiI from the pons shows staining in layer V (Molyneaux, BJ. et al., 2005).
Moreover, overexpression of Fezl has been shown to induce expression of Ctip2 in neurons that do not normally express this transcription factor. On the other hand, even though upon Fezl overexpression the CST can be rescued and the UL neurons can acquire altered axon targeting, this is done without any effect on Satb2 expression and with no induction of Ctip2 (Chen, B., 2008). The above show that Fezl regulates a fate switch between subcortical and callosal projection neurons and that it
does so, both by inducing the expression of Ctip2 as well as through a Ctip2 independent pathway.
Fig. 9. Absence of subcerebral projection neurons in the Fezl-/- mice. A., D., B. and E depict anterograde labeling in wt and Fezl mutants showing the presence of corticothalamic and the absence of subcerebral projections (D) in the Fezl-/- that never reach the spinal cord (E). C and F. Retrograde labeling from the S.C. shows the absence of layer V neuronal labeling in the mutant cortex (Molyneaux, BJ. et al., 2005).
Both Fez and Fezl are expressed in the developing forebrain in overlapping and non-overlapping patterns. During early forebrain development Fez and Fezl have been shown to control the differentiation of progenitor cells by repressing the transcription factor Hes5 thereby alleviating its repression on neurogenin2 (Shimizu, et al., 2010). Recently the transcription factor Tbr1 (expressed primarily in layer VI corticothalamic projection neurons) was shown to directly bind Fezl and repress its expression. In Tbr1 deficient mice there is an ectopic expression of Fezl in Tbr1- cells and a misprojection of these neurons to form CST. Conversely, ectopic expression of Tbr1 in layer V neurons abolishes Fezl expression and CS tract formation, whereas overepxression of Tbr1 in Fezl deficient layer V neurons switches their identity into becoming corticothalamic neurons (Han, W. et al., 2011; McKenna, W.L. et al., 2011).
In the Fezl mutant mice, a recent report has shown a reduced number of layer V and an increased number of UL interneurons. This phenotype was accompanied by restricted neuronal activity in the DL since these neurons, which now acquire a partial
callosal identity, do not recruit the interneurons appropriate for DL where there is normally a greater inhibition of neuronal activity (Lodato, et al., 2011).