Leucine rich repeat kinase 2 (LRRK2) is a 286 kDa protein expressed in a variety of tissues and cell types, including neuronal tissue and innate immune cells. Mutations in LRRK2 have been linked to inflammatory diseases, most notably Crohn’s disease and Parkinson’s disease. Further to this, LRRK2 expression is induced by innate immune stimuli, and can be phosphorylated by Myd88 directed TLR signalling.

Functional experiments were performed using macrophages from WT and LRRK2 knockout mice. Many phenotypes and interactions have been described for LRRK2 in a neuronal or in vitro context; therefore experiments in macrophages were specifically designed to investigate these phenotypes and interactions in an innate immune context. LRRK2 interacts with a range of small GTPase proteins called Rabs, which coordinate and carry out vesicular trafficking, including that of innate immune receptors. Further interactions have been shown with clathrin-mediated endocytic machinery and phagocytic machinery; including cytoskeletal components actin and tubulin. Accordingly, the role of LRRK2 in the expression, membrane localisation, and ligand-induced endocytosis of the innate immune receptors such as TLR4 were assayed. TLR4 plays an important role in immune responses to alpha-synuclein, an immunogenic protein aggregate that accumulates as part of Parkinson’s disease pathology, making it a particularly interesting target for this assay. No effect was shown for LRRK2 on TLR4 expression or receptor mediated endocytosis, so attention was focused upon LRRK2 cytoskeletal interactions. An unclear role of LRRK2 has been described in phagocytosis. Application of LRRK2 KO macrophages in a series of systematic phagocytosis assays was used to demonstrate and clarify that there is no role of LRRK2 in the phagocytosis of simple beads, opsonised material, or complex bacterial targets expressing a range of immunogenic molecules such as LPS.

A genome wide approach was applied to further investigate the role of LRRK2 in TLR4 mediated signalling, as well as NOD2 mediated signalling. Comparison of LPS responses between WT and LRRK2 KO genotype macrophages identified a role of LRRK2 in modulating transcription of a range of chemokines and chemokine receptors. This indicates a specific role of LRRK2 in regulating chemotaxis in LPS stimulated cells. Knockout of LRRK2 resulted in a complete reversal of the regulation of the expression of EPAC1, a cAMP inducible protein working in parallel with a previously described LRRK2 interacting protein PKA. EPAC1 acts, at least in part, via Ca2+ signalling. Modulation of signalling through pathways such as Ca2+, Wnt and cAMP appear as a theme in results described in this transcriptomic experiment. A parallel metabolomic approach allowed analysis of ceramide levels in resting and innate immune stimulated macrophages. Ceramides are lipid molecules able to activate the NLRP3 inflammasome, as well as modulate alpha-synuclein pathology via ceramide metabolomic products. In contrast to results described in neuronal tissue, LRRK2 has no effect on ceramide levels in resting macrophages, however stimulation of NOD2 via MDP resulted in a dramatic LRRK2 specific increase in ceramide levels. Together, these results indicate a role of LRRK2 in activated innate immune cells.