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Leucine-rich repeat kinase 2 (LRRK2), a multifunctional kinase linked to Parkinson’s disease (PD), has been repeatedly implicated in mycobacterial infection, although it’s function outside the nervous system is poorly understood. To create a replicative niche within the harsh environment of a macrophage, Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), needs to manipulate two major innate immune pathways, the type I IFN response and regulated cell death (RCD). Previously, we found that loss of LRRK2 (Lrrk2−/−) in macrophages results in type I IFN defects during Mtb infection in vitro and in vivo. We mechanistically linked this type I IFN reprogramming to LRRK2-dependent mitochondrial dysfunction and leakage of mtDNA into the macrophage cytosol. To further investigate the role of LRRK2 in Mtb pathogenesis, we shifted focus to the G2019S variant of LRRK2, which is the greatest genetic risk factor for PD. While the type I IFN response is normal in Lrrk2G2019S macrophages, their mitochondrial network is fragmented and depolarized. These macrophages also are more prone to a pyroptotic form of regulated cell death (RCD) that is dependent on mtDNA. Importantly, during Mtb infection in vivo, Lrrk2G2019S mice experience hyperinflammation and elevated signatures of pyroptotic cell death. Collectively, these data suggest that Lrrk2G2019S impairs mitochondrial health and enhances inflammatory cell death (pyroptosis) with severe in vivo consequences during Mtb infection. The potent enhanced inflammatory response present in the periphery of G2019S mice in vivo might begin to suggest a model where increased inflammation in the periphery could trigger or exacerbate neurodegeneration in carriers of this allele.

Repeated injection of urethane (ethyl carbamate) is carcinogenic in susceptible strains of mice. Most recent cancer studies involving urethane-induced tumor formation use p53(+/-) mice, which lack one copy of the p53 tumor suppressor gene. In contrast, the same protocol elicits at most a single tumor in wildtype C57BL/6 mice. The effect of repeatedly injecting urethane as a component of a ketamine-xylazine anesthetic mixture in the highly prevalent mouse strain C57BL/6 is unknown. Male C57BL/6J mice (n = 30; age, 3 mo) were anesthetized once monthly for 4 mo by using 560 mg/kg urethane, 28 mg/kg ketamine, and 5.6 mg/kg xylazine. The physical health of the mice was evaluated according to 2 published scoring systems. The average body condition score (scale, 1 to 5; normal, 3) was 3.3, 3.3, and 3.4 after the 2nd, 3rd, and 4th injections, respectively. The visual assessment score was 0 (that is, normal) at all time points examined. Within 1 wk after the 4th injection, the mice were euthanized, necropsied, and evaluated histopathologically. No histopathologic findings were noteworthy. We conclude that repeated monthly injection with urethane as a component of an anesthetic cocktail does not cause clinically detectable abnormalities or induce neoplasia in C57BL/6J mice. These findings are important because urethane combined with low-dose ketamine, unlike other anesthetic regimens, allows for accurate recording of neuronal activity in both the brain and retina. Longitudinal neuronal recordings minimize the number of mice needed and improve the analysis of disease progression and potential therapeutic interventions.

The use of genetic association studies has helped us appreciate that inflammation and immune dysfunction play an integral role in the pathology of multiple chronic disorders including cancer, type II diabetes, and neurodegenerative disease. We became interested in Leucine-rich repeat kinase 2 (LRRK2), a multifunctional kinase associated with Parkinson’s disease (PD), when it was revealed that LRRK2 polymorphisms are also associated with bacterial susceptibility to Mycobacterium leprae, a close relative of Mycobacterium tuberculosis, both of whose pathology relies heavily upon type I interferon (IFN), an innate immune response that activates effector cells. The regulation of type I IFN is critical to prevent a systemic inflammatory state and for the maintenance of healthy neurons. We therefore hypothesized that LRRK2 has a role in regulating innate immunity, contributing to both PD pathogenesis and bacterial infection. We found that in the absence of LRRK2 (Lrrk2−/−), macrophages failed to produce type I IFN in response to multiple cellular insults including, bacterial infection, mitochondrial damage, and DNA/RNA transfection. Interestingly the defect in type I IFN was a result of an imbalance at the level of the mitochondria, which altered macrophage homeostasis. These data suggest that LRRK2 is critical component of the innate immune system through its role maintaining mitochondrial health. The fact that LRRK2-deficient cells cannot induce type I IFN has major implications on both the maintenance of a proper cytokine milieu in the brain and management of the body’s immune response to infection.