PgmNr M5067: Linear-(de)ubiquitination – a (uro)chordate specific mechanism - regulates Wnt signaling in the mouse.

Authors:
S. Ivantsiv 1,2 ; S. Almeida 1,2 ; R. Niibori 2 ; W. Dunham 1,2 ; A. C. Gingras 1,2 ; S. Cordes 1,2


Institutes
1) University of Toronto, Department of Molecular Genetics., Toronto, ON, Canada; 2) Lunenfeld Tanenbaum Research Institute, Mount Sinai Hospital., Toronto, ON, Canada.


Abstract:

Ubiquitination is a post-translational modification that is crucial for many physiological processes and when disrupted, can lead to human diseases, including autism and neurodegenerative disorders. During ubiquitination, a covalent bond is generated usually between the carboxy-terminal amino acid of ubiquitin and a lysine in the target protein. Ubiquitin itself contains seven lysines that can undergo ubiquitination to form polyubiquitin “chains”. An atypical linear ubiquitin chain (called Met1-Ub), which involves fusing the carboxy-terminus of one ubiquitin to the starting methionine of another, emerged in (uro)chordates. Only the linear ubiquitin assembly complex (LUBAC) is known to have linear ubiquitinating activity to date.  We identified the first known dedicated linear deubiquitinase Otulin (Gumby/Fam105b). Prior to our work, linear ubiquitination was only known to have postnatal roles in inflammation and adaptive immunity. By analyses of an allelic series of Otulin mouse mutants, we showed that linear (de)ubiquitination regulates facial nerve and craniofacial development and angiogenesis.

The OTULIN gene resides on human chromosome 5p15.2 (mouse Chromosome 15), deletions of which are associated with craniofacial deficits, intellectual disability and behavioral anomalies seen in Cri du Chat Syndrome (CdCS) patients. By pursuing an interaction between the Wnt signaling component disheveled 2 and OTULIN, we discovered a role for linear ubiquitination in modulating canonical Wnt signaling.  In brief, linear ubiquitination inhibits, while deubiquitination (re)activates canonical Wnt signaling.  Disruption of Wnt signaling can cause craniofacial anomalies, neural tube defects and neuronal dysfunction, and, thus, may lie at the heart of some CdCS symptoms.

To analyze the roles of linear (de)ubiquitination homeostasis in Wnt signaling and of relevance to CdCS, we have generated a conditional Otulin null mutant, which exhibits more severe neurobiological deficits than the extant Otulin point mutants. Moreover we have used AP-MS and the linear ubiquitin binding domain (UBAN) of the NEMO protein fused to glutathione-binding protein (UBAN-GST) to recover OTULIN client proteins. Using this approach we show that linear (de)ubiquitination of key Wnt signaling components controls their subcellular fate and thus regulates Wnt signaling.

This work highlights a new chordate-specific level of regulation of WNT signaling. Furthermore OTULIN belongs to a family of pharmaceutically targetable deubiquitinases and thus may present a novel target for modulating WNT dependent processes relevant to human disorders.