Scientific studies suggest that the leukotriene signaling pathway, an inflammatory pathway originally known for its role in asthma, is involved in the pathogenesis of various neurodegenerative diseases including Parkinson’s disease (PD). We evaluated whether blocking of leukotriene signaling using the approved anti-asthmatic drug Montelukast (MTK) improves motor impairments in the Line 61 mouse model of PD. Indeed, we observed improvements of motor coordination and balance as assessed by the beam walk test in Line 61 mice after 10 weeks of daily oral MTK treatment compared to vehicle-treated Line 61 mice (for details see March 2021 newsletter).
To evaluate cellular changes that might explain the improvements in motor function, brains of the study animals were analyzed histologically. In both, the cerebellum and the caudate putamen, larger microglial soma size in Line 61 mice than in non-transgenic littermates could be observed (Figure 1 A, D), indicating activation of microglia. More interestingly, MTK-treated Line 61 mice show a significant reduction of microglial soma size compared to vehicle-treated Line 61 mice in both brain regions (Figure 1 A, D). In addition, cerebellar microglia of MTK-treated Line 61 animals present significantly longer filaments and more branching points than vehicle-treated Line 61 animals (Figure 1 B, C, E, F).
Figure 1: Microglia phenotype in the cerebellum (CB) and caudate putamen (CPu) of vehicle-treated or Montelukast (MTK)-treated non-transgenic (ntg) and Line 61 mice. Microglial soma size (A, D) was measured using ImageJ. Filament length (B, E) and branching points (C, F) were assessed using the IMARIS software. A total of 15 microglia cells was analyzed per animal (5 animals per group; microglia n = 75 per group). Mean ± SD; one-way ANOVA followed by Bonferroni’s post hoc test compared to the vehicle-treated Line 61 group or Kruskal-Wallis test followed by Dunn’s post hoc test compared to the vehicle-treated Line 61 group; **p<0.01, ***p<0.001.
These observations suggest that MTK treatment induces soma size reduction and filament branching in microglia. Small cell bodies as well as a highly ramified morphology are characteristics of microglia in their homeostatic state. In conclusion, the shift of microglia from a previously activated state to a more homeostatic state might play a central role in driving motor function recovery of Line 61 mice. This is further supported by the fact that the effects of MTK on microglia are especially prominent in the cerebellum, an important brain area for the control and coordination of movement.
Figure 2: Representative images of microglia filaments in the cerebellum of a vehicle-treated and a Montelukast (MTK)-treated Line 61 animal. The left panels show the IMARIS 3D image visualization of Iba1 labeling (white) representing microglia and the right panels show microglia filaments after semi-automatic detection using the filament tracer tool. Blue ball points represent the starting points of filaments (cell bodies); red ball points indicate branching points. Scale bar: 50 µm.