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Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson’s disease. We created a LRRK2 transgenic mouse model that recapitulates cardinal features of the disease: an age-dependent and levodopa-responsive slowness of movement associated with diminished dopamine release and axonal pathology of nigrostriatal dopaminergic projection. These mice provide a valid model of Parkinson’s disease and are a resource for the investigation of pathogenesis and therapeutics.
LRRK2 is the causative gene for PARK8 type Parkinson’s disease with autosomal dominant inheritance1,2. Mutations in LRRK2 are the most common genetic cause of familial and nonfamilial Parkinson’s disease3–5. Most individuals with LRRK2 mutations have dopamine neuron loss in substantia nigra and Lewy body pathology, typical features of idiopathic Parkinson’s disease6. Notably, some individuals with LRRK2 mutations also have tauopathy in the absence of Lewy pathology. Tauopathy and hyperphosphorylated tau are common features of several neurodegenerative diseases7,8. The pathogenesis of Parkinson’s disease resulting from LRRK2 mutations and the more common sporadic forms remains elusive, and a major obstacle to research has been the lack of valid mouse models that recapitulate some of hallmarks of the Parkinson’s disease phenotype9.
To generate LRRK2 transgenic mouse models, we obtained a human bacterial artificial chromosome (BAC) clone containing full length LRRK2 and verified it by sequencing (Supplementary Methods online). The BAC clone was engineered and confirmed to carry R1441G, a common missense mutation in large family pedigrees and with nearly complete penetrance1,2 (Supplementary Figs. 1–3 online).
Both wild-type and LRRK2R1441G transgenic mice were generated (Supplementary Fig. 4 online). Among independent transgenic mouse lines, the RP135 and RP57 lines expressed LRRK2R1441G approximately five- to tenfold above the level of endogenous mouse Lrrk2 (Supplementary Fig. 5 online). Transgene expression was detected in the cortex, cerebellum, striatum and ventral midbrain (Supplementary Fig. 5), consistent with previous reports10. The LRRK2R1441G mice developed normally. At 3 months of age, there was no deficit in body weight, brain weight (data not shown) or motor activity (Fig. 1a).
The LRRK2R1441G mice, but not nontransgenic littermate controls, had age-dependent and progressive motor-activity deficits, beginning with reduced mobility that was reminiscent of hypokinesia in Parkinson’s disease. By 10–12 months of age, the hypokinesia in most LRRK2R1441G mice had progressed to a visually apparent immobility, reminiscent of akinesia in late Parkinson’s disease, in the absence of weakness. These motor deficits in 10–12-month-old LRRK2R1441G mice were observed in both home cage activity (Supplementary Video 1 online) and open field (Supplementary Videos 2 and 3 online), and were confirmed and quantified by the cylinder test (Fig. 1a and Supplementary Videos 4 and 5 online). Levodopa and a direct-acting dopamine agonist, apomorphine, both reversed the deficits in the above-mentioned tasks in mice with severe motor deficits (Fig. 1b and Supplementary Videos 6–8 online). Thus, the LRRK2R1441G transgenic mice faithfully recapitulated the progressive motor deficits and responsiveness to levodopa that are characteristic of human Parkinson’s disease.
We also generated transgenic mice that overexpressed wild-type LRRK2 (WT-OX). The transgene expression in WT-OX mice was comparable to that in LRRK2R1441G mice (Supplementary Fig. 6 online). The WT-OX mice developed normally and did not demonstrate progressive motor behavioral deficits (Supplementary Fig. 6). Therefore, the phenotypes that we observed in LRRK2R1441G mice were the results of the mutation rather than an overexpression of the LRRK2 protein.
To explore whether the hypokinesia in LRRK2R1441G mice was associated with a deficiency of striatal dopamine as in human Parkinson’s disease, we carried out intrastriatal microdialysis in awake and freely moving mice. In the presence of the dopamine re-uptake blocker nomifensine, we observed a significant reduction of extracellular dopamine in LRRK2R1441G mice (P = 0.03; Fig. 2), indicating that dopamine release was indeed impaired.
In histological analysis, there was no observable cell death or gliosis in the spinal cord, and we found no general abnormalities in brain structures in 9–10-month-old LRRK2R1441G transgenic mice (data not shown). Immunohistochemistry for tyrosine hydroxylase in 9–10-month-old LRRK2R1441G mice revealed that dopaminergic neurons in the substantia nigra pars compacta (SNpc) (A9) and ventral tegmental area (A10) were normal in number and anatomical organization (Fig. 3a and Supplementary Methods). We identified two abnormalities of the SNpc dopamine neurons: a decrease (8%, P = 0.01) in average cell body size and a marked diminution in the number of tyrosine hydroxylase–positive dendrites (29%, P = 0.007) in the substantia nigra pars reticulate (Fig. 3a and Supplementary Methods).
Although the overall pattern and optical density of tyrosine hydroxylase immunostaining in the striatum of LRRK2R1441G mice appeared normal (Fig. 3b and Supplementary Methods), profound abnormalities were observed at the cellular level in LRRK2R1441G mice, but not in nontransgenic littermate controls or WT-OX mice. In LRRK2R1441G mouse striatum and piriform cortex, two areas that were enriched in dopaminergic projections, tyrosine hydroxylase–positive axons appeared to be beaded and fragmented and exhibited spheroids and dystrophic neurites (Fig. 3c,d and Supplementary Methods), as described in other forms of axonal injury11–13. Axonal pathology of fragmentation and spheroid formation has been observed in the striata of individuals with Parkinson’s disease14.
Given that tauopathy has been identified in the brains of individuals with Parkinson’s disease resulting from LRRK2 mutations, we performed immunohistochemistry with the AT8 monoclonal antibody, which recognizes a phosphorylated tau epitope15. In dorsal striatum and piriform cortex of LRRK2R1441G mice, but not in nontransgenic littermate control or WT-OX mice, we observed AT8 immunopositive abnormal axonal swellings and dystrophic neurites, similar to those defined by tyrosine hydroxylase staining (Fig. 3d). Furthermore, biochemical analysis revealed that tau is hyperphosphorylated in brain tissues of LRRK2R1441G transgenic mice (Fig. 3e).
LRRK2R1441G BAC transgenic mice offer, for the first time, a model of Parkinson’s disease, derived from a known genetic cause, that successfully recapitulates the motor behavioral, neurochemical and histopathological features of the human disease. These mice will be a powerful tool for in vivo mechanistic studies and therapeutic development.
We wish to thank K. Merchant and her team at Eli Lilly and Company for generating the immunoblot data that facilitated selection of transgenic lines, S. Fleming and M.-F. Chesselet for advice on behavioral studies, and X. William Yang and A. Yamamoto for insightful discussions. This work was supported in part by grants from the Michael J. Fox Foundation and the US National Institutes of Health to C.L., from the Department of Defense to M.F.B., and from US National Institutes of Health and the Parkinson’s Disease Foundation to R.E.B. and T.F.O.
Note: Supplementary information is available on the Nature Neuroscience website.
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