Details

Autor(en) / Beteiligte

• Drinjakovic, Jovana

Titel

E3 ligase nedd4 regulates axon branching by downregulating pten

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2009

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• The goal of the work presented here is to expand our understanding of the molecular mechanisms that regulate axon branching. Retinal ganglion cells (RGCs) send their axons from the retina into the tectum, where they branch and synapse with tectal neurons. A number of guidance cues and their receptors have been described to regulate RGC axon growth and branching. Our group recently demonstrated that guidance cues elicit local activation of the ubiquitin proteasome system (UPS). The UPS targets proteins for degradation: specific E3 ligases conjugates polyubiquitin chains onto their corresponding substrates, which are then recognised by the 26S proteasome and degraded. I found that inhibiting UPS in RGCs by expressing dominant negative ubiquitin mutant that cannot form polyubiquitin chains decreases their branching in the tectum. The E3 ligase Nedd4 is present in RGCs, and RGC axons that contain dominant negative Nedd4, or Nedd4 morpholino, branch less than normal suggesting that Nedd4 is a key E3 regulating branching in RGCs in response to cues. I further showed that a key downstream target of Nedd4 is PTEN, whose levels in the growth cone are Nedd4-dependent, and whose ectopic expression inhibits axon branching. Finally, Netrin-1, which promotes axon outgrowth, induces rapid degradation of PTEN in the growth cones. PTEN is a well established negative regulator of phosphoinositide-3-kinase (PI3K) signalling, whose activation generally promotes neurite extension by increasing actin polymerisation and microtubule stability. Thus, the results of this study suggest a model where Netrin-1 promotes RGC axon branching by inducing Nedd4-mediated degradation of PTEN, thus leading to upregulation of PI3K signals, and consequent cytoskeletal rearrangements that bring about branch formation.