Exactly a year ago, the LRRK2 gene was reported to cause inherited cases of Parkinson disease (see ARF related news story) and, a few months later, mutations in sporadic forms of PD followed suit (see ARF related news story). In just this past year, 48 papers appeared on this gene, most of them reporting mutations. Simultaneously, scientists pounced on the study of LRRK2 function and its role in pathology. This will be an extended inquiry—dardarin is a massive gene with 51 exons—but this week, initial results on the functional front started rolling in.
LRRK2 stands for leucine-rich repeat kinase 2, and the gene also goes by the name of dardarin. As with many new discoveries, it has raised many questions. Why, to name but one, does the same dardarin mutation cause formation of intracellular inclusion bodies in some patients but not others? Some have speculated that because this large protein has a kinase domain, its pathology may result from its ability to phosphorylate different substrates. However, no one has demonstrated that dardarin mutations affect its kinase activity—until now.
In a paper to be published this week in the online PNAS Early Edition, Valina and Ted Dawson, with colleagues at Johns Hopkins University School of Medicine, Baltimore, Maryland, report that two familial Parkinson disease (PD) mutations in dardarin, one in its mixed-lineage kinase (MLK)-like domain and one in its GTPase domain, increase the kinase activity of the protein. The finding is compatible with dardarin’s autosomal-dominant mode of inheritance and supports the suggestion that phosphorylation plays a key role in PD pathology, the authors write.
The authors first asked if dardarin mutations might disturb the subcellular localization, turnover, or steady-state levels of the protein. Using confocal microscopy, they found most wild-type and mutant protein to be diffusely distributed throughout the cytoplasm and some to be associated with the outer membrane of the mitochondria. Using antibodies to quantify the amount of protein in dopaminergic neuroblastoma cells (SH-SY5Y cells), they found that dardarin was ubiquitinated, but that protein harboring either the GTPase or MLK mutations was degraded at the same rate as the wild-type protein.
It was in tests of kinase activity where first author Andrew West and colleagues found major differences between wild-type and mutant protein. Using myelin basic protein (MBP) as a substrate, the authors found that the MLK mutant phosphorylated almost three times more substrate than did wild-type dardarin. Perhaps surprisingly, they also found that the GTPase domain mutant modified almost twice as much MBP as did wild-type dardarin, though this result was not statistically significant. A test for autophosphorylation returned a slightly different result; in this scenario, both mutants were significantly more active than wild-type protein.
Why a mutation in the GTPase domain increases dardarin kinase activity is uncertain. The authors point out that many kinases associate with activated GTPases, and they suggest that the GTPase domain within dardarin might serve an autoregulatory role. If additional experiments and independent confirmation indeed show increased kinase activity to be at the heart of dardarin-mediated pathology, this might be a hopeful sign. “Our findings suggest that development of agents that abrogate LRRK2 kinase activity might offer therapeutic potential for the treatment of PD," the authors write. Until then, many questions remain, not the least being, what are the natural substrates for dardarin’s kinase activity, and which pathways do they drive? For experimental purposes, the authors used the generic MBP to test the kinase. They also tested parkin, tau, and α-synuclein—three proteins inextricably linked to PD—and found that none of them are substrates for dardarin.—Tom Fagan
No Available Further Reading
- West AB, Moore DJ, Biskup S, Bugayenko A, Smith WW, Ross CA, Dawson VL, Dawson TM. Parkinson's disease-associated mutations in leucine-rich repeat kinase 2 augment kinase activity. Proc Natl Acad Sci U S A. 2005 Nov 15;102(46):16842-7. PubMed.