Browsing by Subject "Receptors, Eph Family"
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Item EphB: Ephrin-B Bidirectional Signaling in Retinocollicular Mapping(2011-02-01T19:35:06Z) Thakar, Sonal Gautam; Henklmeyer, MarkRetinal ganglion cell axon exit into the optic disc and formation of the optic nerve and termination in the superior colliculus is critical for integrating both the visual field and head orientation to mediate appropriate eye movement. Expression of EphB receptor tyrosine kinases and ephrin-B ligands in the visual system signify the potential role for EphB:ephrin-B bidirectional signaling in the development of the visual system. Previous studies of EphB2-/-;EphB3-/- (receptor null mutant) and EphB2lacZ/lacZ;EphB3-/- (EphB2 intracellular signaling deficient mutant) mice established that EphB acting as a ligand mediates RGC axon exit from the optic disc. Use of these compound mutants also showed EphB2 forward signaling mediates ventral RGC axon branching and formation of correct termination zones (TZ) within the medial SC. However, the molecules acting as the receptor to mediate dorsal RGC axon exit into the optic disc are unknown. Also, the EphB2 intracellular component essential for retinocollicular mapping is unknown as are the roles for EphB1, ephrin-B1, and ephrin-B2, which are also expressed in the visual system. I found that ephrin-B1 and ephrin-B2 reverse signaling has a minor role for directing RGC axon guidance to the optic nerve head. On the other hand, EphB:ephrin-B bidirectional signaling appears to play a major role in dorsoventral RGC axon retinocollicular mapping. DiI labeling of a subset of dorsal or ventral-temporal RGC axons allowed visualization of the termination zone in the superior colliculus in postnatal pups. After a comprehensive analysis of various EphB and ephrin-B null, intracellular truncation, and point mutants, that affect specific components of forward and reverse signaling, EphB:ephrin-B bidirectional signaling was found to be the key mediator of dorsoventral retinocollicular mapping. Specifically, EphB2 tyrosine kinase catalytic activity alone is critical for ventral RGC axon retinocollicular mapping and EphB1 forward signaling is crucial for ventral RGC axon retinocollicular mapping. Together, EphB1 and EphB2 are the chief mediators of ventral RGC axon retinocollicular mapping. Whereas ephrin-B1 expressed in the SC acts as a ligand to mediate ventral RGC axons, ephrin-B2 expressed in a high dorsal/low ventral gradient in the retina functions as a receptor that is important for both dorsal and ventral RGC axon retinocollicular mapping. Establishing the molecules involved with retinocollicular mapping will focus the analysis of the remaining questions pertaining to the development of the visual system.Item The Intracellular Domain of EPHB1 Is Required For Axon Pathfinding at the Optic Chiasm and Corpus Callosum(2011-08-26T17:34:18Z) Chenaux, George; Henkemeyer, MarkThis dissertation presents evidence of the importance of EphB1 mediated signaling in retinal and callosal axons while attempting to reach their targets. EphB receptor tyrosine kinases direct axonal pathfinding through interactions with ephrin-B proteins following axon-cell contact. Since EphB:ephrin-B binding leads to bidirectional signals, the contributions of signaling into the Eph-expressing cell (forward signaling) or the ephrin-expressing cell (reverse signaling) cannot be assigned using traditional protein-null alleles. To determine if EphB1 is functioning as a receptor during axon pathfinding, I created a new knock-in mutant mouse, EphB1 T-lacZ, that expresses an intracellular-truncated EphB1-β-gal fusion protein from the endogenous locus. As in the EphB1 -/-protein-null animals, the EphB1 T-lacZ/T-lacZ homozygotes fail to form the ipsilateral projecting subpopulation of retinal ganglion cell (RGC) axons. This indicates that forward signaling through the intracellular domain of EphB1 is required for proper axon pathfinding of RGC axons at the optic chiasm. Further analysis of other EphB and ephrin-B mutant mice shows that EphB1 is the preferred receptor of both ephrin-B1 and ephrin-B2 in mediating axon guidance at the optic chiasm despite the coexpression of EphB2 in the same ipsilaterally projecting RGC axons. In addition to analyzing the axon pathfinding defect at the optic chiasm, the EphB1 T-lacZ mice were also used to analyze another phenotype associated with EphB1 -/-protein-null animals, a failure to properly form a corpus callosum. I will show that the intracellular domains of EphB1 and EphB2 are important for the guidance of callosal axons across the midline during the formation of the corpus callosum. However, opposite to the above mentioned optic chiasm phenotype, these animals have axons that fail to project contralaterally choosing to remain on the ipsilateral hemisphere.