The Role of the E3 Ubiquitin Ligase Cdh1-APC in Axon Growth in the Mammalian Brain

Die Rolle der E3 Ubiquitin Ligase Cdh1-APC in Axon Wachstum im Gehirn von Säugetieren

by Madhuvanthi Kannan
Doctoral thesis
Date of Examination:2012-08-22
Date of issue:2012-09-14
Advisor:Dr. Judith Stegmüller
Referee:Dr. Judith Stegmüller
Referee:Prof. Dr. Gregor Bucher
Referee:Dr. Dr. Oliver Schlüter
Persistent Address: http://dx.doi.org/10.53846/goediss-1449

 

 

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Abstract

English

Neuronal differentiation begins with initiation of the axon. Both extrinsic factors in the neuronal environment and cell-autonomous players contribute to successful axonal morphogenesis. The E3 ubiquitin ligase, Cdh1-Anaphase Promoting Complex (Cdh1-APC) is implicated in cell-intrinsic inhibition of axonal growth in the mammalian brain. In restricting axon growth, Cdh1-APC targets the transcription factors SnoN and Id2 for proteasomal degradation. However, whether the E3 ligase collaborates with extrinsic signaling in controlling axon growth is unknown. In this study, I show that mitogen activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K) and Rho GTPases, effectors of extrinsic trophic signaling, interface with the Cdh1-APC pathway. While MAPK and PI3K activity antagonize Cdh1 inhibition of axon growth, the small Rho GTPases RhoA and Cdc42 are positive mediators of the pathway. A mechanistic insight into the Cdh1-APC/RhoA relationship identified the homologous to E6-AP (HECT) E3 ligase, smad ubiquitination regulatory factor-1 (Smurf1), a negative regulator of RhoA, as a substrate of Cdh1-APC. Smurf1 promotes axon growth downstream of Cdh1-APC by targeting RhoA to proteasomal degradation. Smurf1 binds to Cdh1-APC in a D-box dependent manner and consequently, a stable Smurf1 D-box mutant overcomes Cdh1 inhibition by degrading RhoA and exhibits a gain-of-function in axon growth both on permissive substrates and on myelin. p250 GTPase activating protein (p250GAP), a RhoA inactivator, also participates in the Cdh1-APC pathway alongside Smurf1. Again a positive regulator of axon growth, p250GAP shares a linear pathway downstream of Cdh1-APC. Taken together, my study places Cdh1-APC at the intersection of intrinsic and extrinsic signaling cascades that controls axon growth. Further, my results reiterate an emerging role for the E3 ligase as a suppressor of axon extension and uncover novel candidates for regenerative therapy.
Keywords: Cdh1-APC; RhoA; Smurf1; p250GAP; ubiquitination; axon growth; regeneration

Other Languages

Mit der Ausbildung des Axons beginnt die neuronale Differenzierung. Sowohl extrinsische Faktoren der zellulären Umgebung als auch intrinsische Programme tragen zur erfolgreichen Axonogenese bei. Die E3 Ubiquitin Ligase Cdh1-APC (Anaphase Promoting Complex) ist ein zellintrinsischer Inhibitor des Axonenwachstums im Säugergehirn. In diesem Zusammenhang sorgt Cdh1-APC für den proteasomalen Abbau der Transkriptionsfaktoren SnoN und Id2. Jedoch ist unklar, ob Cdh1-APC mit extrinsischen Signalen kooperiert, um Axonwachstum zu steuern. In dieser Studie zeige ich, dass MAPK (mitogen activated protein kinase), PI3K (phosphatidylinositol-3-kinase) und Rho GTPases, Effektoren extrinsischer trophischer Signale, mit dem Cdh1-APC Signalweg interagieren. Während MAPK und PI3K der Inhibition von Cdh1-APC entgegenwirken, unterstützen die GTPasen RhoA und Cdc42 die Wachstumsinhibition von Cdh1-APC. Mechanistischen Einblick gewährte die Identifizierung der HECT (homologous to E6-AP) E3 Ligase Smurf1 als Substrat von Cdh1-APC. Smurf1 stimuliert Axonenwachstum
Schlagwörter: Cdh1-APC; RhoA; Smurf1; p250GAP; Ubiquitination; Axonenwachstum; Regeneration
 
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