Identification of b-catenin and other RNAs in developing thalamic axons
Davey, John William
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This thesis provides evidence for the presence of multiple RNAs in the axons and growth cones of developing thalamic cells, particularly the mRNA for the cell adhesion and Wnt-signalling-related molecule b-catenin. After many decades of effort, mRNAs have been shown to be present in the axons of many different systems in recent years. Furthermore, these mRNAs have been shown to be locally translated at the growth cone, and this local translation is required for axons to turn in response to multiple guidance cues. As studies accumulate, it is becoming clear that different axonal systems contain different complements of mRNAs and have different requirements for local translation. One axonal system which has not been investigated to date is the thalamocortical tract. The nuclei of the thalamus are connected to the areas of the cortex via bundles of axons which travel from the thalamus to the cortex via the ventral telencephalon during embyronic development. These axons make a number of turns and are guided by many cues and other axonal tracts before innervating their cortical target. In this thesis, a quantitative real-time polymerase chain reaction (qRT-PCR) approach is developed to isolate multiple mRNAs from developing thalamic axons in vitro, including b-catenin mRNA, b-actin mRNA, 18S ribosomal RNA and ten other mRNAs. The method used should be suitable for use with other axonal systems and also for testing the effect of guidance cues on mRNA expression in axons. The qRT-PCR results for b-catenin, b-actin and 18S have been validated using in situ hybridisation. Analysis of in situ hybridisation results indicates that b-catenin and 18S, but not b-actin, are upregulated in the growth cone compared to the axon. As b-catenin has been shown to be involved in axon guidance via Slit and ephrin guidance cues in other axonal systems, and these guidance cues act upon thalamocortical axons, the identification of b-catenin mRNA in thalamic axons is an important step towards a full understanding of the thalamocortical system. The results presented here indicate that local protein synthesis is likely to occur in thalamic axons as it does in other axonal systems, and that local translation is likely to be important for thalamic axonal responses to guidance cues and other axonal tracts.