Fbxl18 targets LRRK2 for proteasomal degradation and attenuates cell toxicity

- We identified a novel protein, Fbxl18, that associates with LRRK2 and targets LRRK2 for degradation by the proteasome.
- We demonstrate that phosphorylation of LRRK2 is required for Fbxl18 to bind to LRRK2 and to promote LRRK2 degradation.
- siRNA-knockdown of endogenous Fbxl18 increases levels of endogenous LRRK2 while Fbxl18 overexpression decreases LRRK2.
- Increasing or decreasing Fbxl18 mitigates or enhances LRRK2-mediated cell toxicity.

Dominantly inherited mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common causes of familial Parkinson's disease (PD) and LRRK2 polymorphisms are associated with increased risk for idiopathic PD. However, the molecular mechanisms by which these mutations cause PD remain uncertain. In vitro studies indicate that disease-linked mutations in LRRK2 increase LRRK2 kinase activity and LRRK2-mediated cell toxicity. Identifying LRRK2-interacting proteins and determining their effects on LRRK2 are important for understanding LRRK2 function and for delineating the pathophysiological mechanisms of LRRK2 mutations. Here we identified a novel protein, F-box and leucine-rich repeat domain-containing protein 18 (Fbxl18) that physically associates with LRRK2. We demonstrated that Fbxl18 is a component of a Skp1-Cullin1-F-box ubiquitin ligase complex that regulates the abundance of LRRK2 by selectively targeting phosphorylated LRRK2 for ubiquitination and proteasomal degradation. Knockdown of endogenous Fbxl18 stabilized LRRK2 abundance while protein kinase C activation enhanced LRRK2 degradation by Fbxl18. Dephosphorylation of LRRK2 blocked Fbxl18 association with LRRK2. Taken together, we have identified potential mechanisms for LRRK2 regulation by kinase signaling pathways. Furthermore, Fbxl18 prevented caspase activation and cell death caused by LRRK2 and PD-linked mutant LRRK2. This reveals novel targets for developing potential therapies for familial and idiopathic PD.