Zic 2 promotes axonal divergence at the optic chiasm midline by EphB 1-dependent and-independent mechanisms

INTRODUCTION Most metazoans are bilaterally symmetric and many features of mature neural function, including the interpretation of sensory information and the coordination of locomotion, depend on the coherent communication between the two halves of the nervous system. The mechanism underlying this communication is established by axons of neurons located on one side of the nervous system growing towards, recognizing and then choosing to transverse or avoid the embryonic midline. Thus, alterations in the decision to cross or not to cross the midline may perturb the proper establishment of nervous system circuitry. For instance, in albino animals or in organisms that have abnormalities in the pyramidal tract, perturbations in fiber decussation cause problems in stereoscopic vision and in motor coordination, respectively (Beg et al., 2007; Jeffery, 2001; Kullander et al., 2003). The crossing of the midline has frequently been employed as a model to understand axonal guidance. Studies on how this process occurs have resulted in the identification of a considerable number of specific axon guidance ligand-receptor pairs such as netrins and DCC, Slits and Robos, semaphorins and neuropilins/plexins and ephrins and Ephs, to cite some examples (Erskine et al., 2000; Keleman et al., 2002; Kidd et al., 1998; Lustig et al., 2001; Nakagawa et al., 2000; Oster et al., 2003; Plump et al., 2002; Stein et al., 2001; Williams et al., 2006; Williams et al., 2003; Zou et al., 2000). A new level of analysis of the mechanisms of axonal navigation has revealed that combinations of transcription factors expressed in neuronal subpopulations control diverse trajectories to specific targets (Jessell, 2000; Shirasaki and Pfaff, 2002). How the activity of a specific transcription factor dictates axon guidance choices is still an open question, but in recent years it has become clear that transcription factors provide a molecular drive by which growth cones can respond appropriately and differentially to the cues they encounter through the activation and inhibition of a balance of target axon guidance molecules. Thus, the transcription factor Even-skipped (Eve), expressed in motoneurons, dorsalizes ventral axons in part through upregulation of the netrin receptor Unc-5, which can act with the additional netrin receptor DCC (Labrador et al., 2005). The cell adhesion molecule Fasciclin III might be a potential target for Nkx6 (HGTX – FlyBase) in a subset of ventral motoneurons that require this molecule to exit the central nervous system (Broihier et al., 2004). In vertebrate systems, Lim1 (Lhx1) controls motoneuron projections to the limb in the dorsoventral axis by inducing the expression of EphA4, enabling dorsal projecting axons to respond to ephrin A expressed by the ventral limb mesenchyme (Kania and Jessell, 2003), whereas Islet2 seems to be upstream of Slit-induced fasciculation of sensory neurons (Yeo et al., 2004). In the murine visual system, Irx4 regulates Slit proteins to control axonal navigation within the retina (Jin et al., 2003). The guidance cue Shh and the cell adhesion molecule L1 have been identified as targets of the Pou transcription factor Brn3b (Pou4f2), which is essential for retinal pathfinding at several points along the retinofugal pathway (Erkman et al., 2000; Pan et al., 2005). Misexpression and genetic manipulation of the transcription factors FoxG1, FoxD1, Vax2 or Tbx5 alter regional specification of visual paths in the targets by regulation of axon guidance molecules such as Ephs and ephrins (Herrera et al., 2004; Koshiba-Takeuchi et al., 2000; Mui et al., 2002; Pratt et al., 2004; Schulte and Cepko, 2000; Takahashi et al., 2003). These and other findings describe links between transcription factor expression and regulation of axon guidance proteins in different scenarios of the developing nervous system (for a review, see Erskine and Herrera, 2007). However, to date, there have been no reports on transcriptional regulation of axon guidance molecules acting on laterality at the vertebrate midline. The optic chiasm, a structure essential for establishing binocular vision, is a good model for analyzing axon guidance and midline crossing because at this juncture, retinal fibers arising from each retina diverge to project to the higher visual targets in the same (ipsilateral) or the opposite (contralateral) side of the brain. The turn at the midline executed by ipsilateral axons has been suggested to be mediated by EphB1/ephrin B2 signaling. Ephrin B2 is expressed by glial cells located at the midline, whereas EphB1 is highly expressed in the ventrotemporal (VT) retina where ganglion cells giving rise Zic2 promotes axonal divergence at the optic chiasm midline by EphB1-dependent and -independent mechanisms