The establishment of two BAC transgenic strains expressing similar levels and patterns of Wt LRRK2 and G2019S mutation provides a unique opportunity to dissect the physiological function of LRRK2 and unravel the pathogenic role of LRRK2-G2019S mutation in PD. Our study provides the first in vivo evidence that overexpression of Wt LRRK2 increases striatal dopamine release and enhances motor performance. By contrast, overexpression of LRRK2 variant carrying the most common familial PD mutation G2019S fails to provide these benefits, indicated by an age-dependent decrease in DA content, impairment of evoked striatal DA release and decreased DA uptake in LRRK2-G2019S mice. In addition, LRRK2 G2019S neither enhanced nor compromised motor function in mice up to 12 months. These results, therefore, suggest that LRRK2 regulates striatal DA transmission, thereby facilitating motor activity, and that G2019S mutation may cause an impairment of this function of native LRRK2.
An important finding in our study is that the age-dependent impairment in striatal DA release and loss of DA content in LRRK2-G2019S mice was not associated with obvious neuronal death or nigrostriatal terminal degeneration (by 18-20 months) or motor deficits (by 12 months) in LRRK2-G2019S mice. These data indicate that an initial consequence of G2019S mutation is a functional
rather than physical
denervation of the nigrostriatal DA pathway. Previous studies suggest that LRRK2 is localized at membrane structures, including synaptic vesicles (Hatano et al., 2007
) and may regulate synaptic vesicle proteins and vesicle endocytosis and, thus, vesicle life cycle (Sakaguchi-Nakashima et al., 2007
). Our data showing elevated DA release in LRRK2-Wt mice along with enhanced motor performance are consistent with this LRRK2 function. Conversely, decreased DA release and release sustainability in LRRK2-G2019S mice would be consistent with alteration of this function. Whether the progressive nature of DA impairment is a result of an age-dependent accumulation of LRRK2 protein (Li et al., 2007
) or protein modification of LRRK2 over time remains to be established. Another potential function of LRRK2 is suggested by the observation that LRRK2-Wt overexpression decreases the number of phospho-tau (+) cells, suggesting a protective role of LRRK2 against tau-associated pathology. By contrast, this protection of LRRK2 is absent in G2019S mutant. .
The lack of nigrostriatal degeneration in LRRK2-G2019S mice was initially surprising because of previous in vitro
studies showing that increased kinase activity in LRRK2-G2019S was correlated with enhanced neurotoxicity (Smith et al., 2006
; West et al., 2007
). Despite significantly higher kinase activity of brain LRRK2-G2019S protein than its Wt counterpart, our LRRK2-G2019S mice showed no evidence of the neurodegeneration characteristic of PD. This offers further support for the notion that LRRK2-G2019S mutation can impair LRRK2 neuronal function without activating cell death pathways at early disease stages. Consistent with this, many carriers of PD-related mutations of LRRK2 do not develop PD symptoms. Indeed, an international human genetics study of PD involving >1,000 subjects showed that the penetrance of the G2019S mutation is only 28% by age of 59, increasing to 51% by age 69 (Healy et al., 2008
). Thus, the LRRK2-mediated pathogenic pathway may be particularly susceptible to modification by other genetic or cellular mechanisms. Interestingly, although expression of mutant LRRK2 in drosophila
consistently produces neurodegeneration, co-expression of another PD-related protein parkin can prevent this (Ng et al., 2009
Two recent LRRK2 mouse models expressing R1441G/C mutation also provide evidence for impaired DA transmission (Li et al., 2009
; Tong et al., 2009
). However, neither previous study examined nigrostriatal DA release and uptake dynamics, the effect of normal LRRK2 overexpression, or the link between LRRK2 kinase/GTPase activity to neurotoxicity, as here. Our study further indicates that LRRK2-Wt overexpression decreases the number of phospho-tau (+) cells, suggesting a protective role of LRRK2 against tau-associated pathology that is not observed with the G2019S mutation. Interestingly, although both previous studies examined the R1441C/G mutation, which is a relatively rare mutation in PD, those mice exhibited several dramatic differences in pathology and behavior, some of which also differ from our findings in LRRK2-G2019S mice. Factors contributing to these phenotypic differences may include mutation examined (G2019S vs
. R1441G/C), source species (murine vs
. human), genetic background (C57BL/6J vs
. FVB), and transgene expression levels.
Taken together, these data support previous evidence that alteration of a single PD-related gene, whether autosomal dominant (alpha-synuclein) or autosomal recessive (DJ-1, Parkin, and Pink1), in mouse models is unlikely to recapitulate the full spectrum of PD symptoms. Finally, our study, along with others (Li et al., 2009
), reveals a causal role for the LRRK2 mutant in the early pathogenesis of PD: impairment of striatal DA transmission. Recent PET imaging in presymptomatic individuals carrying the LRRK2 familial mutation suggests dysfunction of striatal DA transmission as an early event in disease progression in PD (Adams et al., 2005
; Nandhagopal et al., 2008
). Moreover, other genetic mouse models for PD also exhibit aberrant DA transmission in the absence of nigrostriatal degeneration (Abeliovich et al., 2000
; Goldberg et al., 2003
; Chen et al., 2005
; Kitada et al., 2007
; Zhou et al., 2008
). Thus the emerging evidence, including the present findings from LRRK2 BAC transgenic mice, suggests a common mechanism for PD pathogenesis whereby familial PD mutations impair striatal DA transmission prior to the degeneration of DAergic neurons.