Id proteins (Inhibitors of differentiation) inhibit differentiation by preventing bHLH DNA binding activity through the formation of inactive Id-bHLH heterodimers. Xenopus Ids (xIds) are dynamically expressed in a wide variety of tissues, with high expression levels detected at early stages of development. In this work, I have identified a requirement for Xenopus xId2 and xId3 in primary neurogenesis and muscle differentiation. Overexpression of xId2 and xId3 disrupts the expression of late neuronal and muscle differentiation markers, but not the expression of the early markers xNGN and xMyoD, suggesting that xIds inhibit primary neuron and muscle differentiation without affecting neuronal or myogenic commitment. To elucidate the mechanism through which xIds inhibit primary neurogenesis, I investigated the ability of xIds to affect xNGN activity, the most upstream activator of neurogenesis that is both necessary and sufficient to initiate neuronal differentiation in vertebrates. I found that xId2 and xId3 negatively regulate xNGN’s activity at a post-transcriptional level, albeit through distinct mechanisms. While both xIds are able to inhibit xNGN’s ability to activate transcription in reporter assays, only xId3 is able to significantly reduce the DNA binding ability of xNGN/E12 heterodimer, through the sequestration of xE12. Importantly, I also demonstrate that by sequestering xE12, xId3 induces xNGN degradation. These findings not only provide the first direct evidence that xId3 modulates xNGN activity and stability, but also emphasize that xId2 and xId3 act through distinct pathways to ensure a stronger short-term control of xNGN activity and consequently prevent premature primary neuron differentiation. This non-redundant role for xId2 and xId3 is further supported by the finding that xId2 and xId3 expression and activity is differentially regulated. Finally, I also provide evidence for a role of Casein Kinase II (CKII) mediated phosphorylation on xNGN activity.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:595480 |
| Date | January 2009 |
| Creators | Almeida, A. A. D. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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