Browsing by Subject "Axons"
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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 Functional analysis of SOX11 and NF1 in sensory neuron development and plasticity(2009-01-14) Lin, Lu; Parada, Luis F.Development of sensory neurons is controlled by both cell-extrinsic and cell-intrinsic factors. The transcription factor Sox11 is abundantly expressed in embryonic sensory neurons. In the first part of this thesis, by using a Sox11-/- mouse model, I show that the loss of Sox11 results in increased cell death in embryonic sensory ganglia, which leads to a significant neuronal loss around birth. The ablation of Sox11 also leads to cell-death-independent axonal growth defects both in vivo and in vitro. Furthermore, I demonstrate that Sox11-deficient MEFs exhibit decreased level of phospho-AKT compared to controls. These data suggest that Sox11 is required for multiple key steps of sensory neuron development, probably through regulating the expression of component(s) of the signaling pathways downstream of growth factors. Adult sensory neurons exhibit various degrees of plasticity following different types of injury. Neurofibromin, the protein encoded by the tumor suppressor gene Nf1, functions as a negative regulator of Ras and its two major downstream pathways: MEK-ERK and PI3K-Akt pathway. Nf1-/- embryonic sensory neurons survive without neurotrophin support attributed to enhanced PI3K activity in the absence of Nf1. In the second part of this thesis, by using mice with Nf1 specifically deleted in neurons (Nf1-SynI-CKO), I demonstrate that Nf1-/- adult sensory neurons exhibit enhanced neurite outgrowth in vitro. After dorsal root injury, spontaneous functional recovery and increased axonal sprouting from uninjured sensory neurons are observed in Nf1-SynI-CKO mice compared to permanent sensory deficits in controls. These appear to be mediated both by cell autonomous capacity of spared Nf1-/- DRG neurons and by non-cell autonomous contribution from Nf1-/- neurons in the spinal cord, as suggested by co-culture experiments. To further confirm whether the spinal cord or DRG neurons contribute to functional recovery in Nf1-SynI-CKO mice, I generated other mutants with Nf1 deleted more specifically and completely in DRG (Nf1-Isl1-CKO). In the third part of this thesis, I discuss some unanticipated results due to Cre expression in the gastrointestinal tract. Nf1-Isl1-CKO mice develop gastric epithelium hyperplasia and inflammation with increased proliferation and apoptosis. These phenotypes seem to be attributed to the loss of Nf1 in non-gastric epithelial cells.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.Item Schwann cell processes guide axons reinnervating the neuromuscular junction(2004) Kang, Hyuno; Thompson, Wesley J.