Parkinson’s disease (PD), a progressive neurodegenerative disorder caused by the loss of dopaminergic neurons, currently affects nearly 7 million people worldwide. Mutations in leucine-rich repeat kinase 2 (LRRK2), a dual functional protein with separate kinase and GTPase domains, are the most common cause of autosomal dominant PD and also contribute to sporadic PD. Multiple lines of evidence indicate that abnormal LRRK2 kinase and GTP domain activities contribute to neurodegeneration in LRRK2-linked PD and that the GTPase domain positively regulates the kinase domain, though the mechanism has not yet been defined.
While rapid progress has been made in the development of LRRK2 kinase inhibitors, issues of non-specificity and low brain penetration have arisen with many of the proposed agents and there are, to date, no published reports of GTPase domain inhibitors for LRRK2. However, a new study from researchers at the University of Maryland, proposes two novel GTPase domain inhibitors. Their results, published in Human Molecular Genetics, suggest that both compounds have the potential for development into therapeutic agents for the treatment of PD.
Using a combination of computer-aided drug design (CADD) and biological screening, the researchers identified two novel compounds, 68 and 70, that reduced LRRK2 GTP-binding and kinase activity. The two compounds reduced the kinase activity of full-length LRRK2 but did not alter the kinase activity of the LRRK2 kinase domain which further supports the idea that kinase activity is regulated by GTP-binding activity and suggests that these inhibitors reduce kinase activity by altering GTPase domain function. The compounds did not alter GTP binding or kinase activity with the closely related LRRK1, suggesting that they are relatively specific to LRRK2. Furthermore, both compounds showed similar effects on GTP binding and kinase activity in cellular assays and, in addition, reduced neuronal degeneration in cultured murine neurons and SH-SY5Y cells.
The effects of the two newly identified compounds are similar to those of the known LRRK2 kinase inhibitor, LRRK2-In-1. However, LRRK2-In-1 cannot pass the blood-brain barrier, thus greatly limiting its effectiveness for the treatment of PD. Using a transgenic mouse model with LRRK2 mutant variants the researchers observed that 68 inhibited GTP binding and kinase activity in the brain, suggesting that 68 can readily penetrate the BBB, one of the greatest obstacles to developing therapeutic compounds for PD and other neurodegenerative diseases. In addition, 68 attenuated LRRK2 mediated microglia activation and inflammation, a known contributor to dopaminergic neuron degeneration.
The researchers believe their findings suggest that disruption of GTP binding to LRRK2 represents a potential novel therapeutic approach for PD intervention and that the newly identified GTP-binding inhibitors provide both tools and lead compounds for future drug development.