|Abstract: ||The mechanisms by which Leucine-Rich Repeat Kinase-2 (LRRK2) mutations in humans are linked to the risk of Parkinson disease (PD), Crohn’s disease and leprosy remain elusive. We hypothesized a shared role for LRRK2 in immune system functions.
I discovered robust LRRK2 expression in mammalian leukocytes, foremost within cells of the innate immune system. For example, human lymph nodes, spleen, and distal ileum exhibited abundant LRRK2-positive macrophages and granulocytes, findings that were confirmed by FACS of cells collected from the same organs of adult mice. Microscopy studies revealed robust LRRK2 reactivity in infiltrating, myeloperoxidase-positive granulocytes and CD68-positive macrophages in inflammatory conditions, such as viral encephalitis, idiopathic radiculitis, terminal ileitis and abscess formation. Analysis of midbrains from idiopathic versus genetic variants of Parkinson disease (PD) revealed rare, anti-LRRK2-positive reactivity that was confined to intravascular leukocytes. Neuronal LRRK2 signals were seen in forebrain regions, consistent with reports in the literature. To explore an immunological role for Lrrk2, we first examined bone marrow-derived macrophages from PD-linked R1441C knock-in mice and wild-type (WT) animals. Following stimulation with bacterial and viral pathogens, Lrrk2 expression was increased in cells from both genotypes.
In subsequent in vivo experiments using an established, nasal inoculation model of newborn mice with a virulent microbe, we detected a role for WT Lrrk2 in modifying disease outcomes, such as after reovirus (type-3 Dearing) infection. There, Lrrk2 deficiency conferred increased vulnerability to elevated viral protein levels in the brain and greater mortality rates from encephalitis in mice. In contrast, we observed an initially protective role for the PD-linked G2019S mutant in the same reovirus inoculation model; there, lower viral titers were recorded in the lungs and brain of acutely infected, Lrrk2 knock-in mice at days 3 and 11 post-inoculation (dpi), respectively. Paradoxically, in related survival studies, we observed a significantly higher (rather than the expected reduced) mortality rate during the ensuing weeks in female mice that carried the G2019S Lrrk2 mutation. Related screening for cytokine dysregulation in infected tissues of mutant mice revealed significant changes in select signaling molecules, e.g., MIG and IP10.
These collective results suggest a role for mammalian LRRK2 in the innate immune system following the encounter of a virulent pathogen, which is associated with a female sex bias. An early mechanistic clue points at changes in cytokine production by infected tissue, a second at the degradation efficiency of viral proteins. We speculate that LRRK2 alleles function in the regulation of the host’s innate response to invading pathogens, which may help explain its association with three human disorders, each of which is pathogenetically associated with one or more environmental trigger. Future studies will test LRRK2’s function in other infection paradigms, expand on mechanisms underlying genotype-dependent differences in inflammation, and determine the effects of LRRK2’s kinase inhibition in in vivo models.|