Targeted Overexpression of α-Synuclein by rAAV2/1 Vectors Induces Progressive Nigrostriatal Degeneration and Increases Vulnerability to MPTP in Mouse

Abstract

Mutations, duplication and triplication of α-synuclein genes are linked to familial Parkinson’s disease (PD), and aggregation of α-synuclein (α-syn) in Lewy bodies (LB) is involved in the pathogenesis of the disease. The targeted overexpression of α-syn in the substantia nigra (SN) mediated by viral vectors may provide a better alternative to recapitulate the neurodegenerative features of PD. Therefore, we overexpressed human wild-type α-syn using rAAV2/1 vectors in the bilateral SN of mouse and examined the effects for up to 12 weeks. Delivery of rAAV-2/1-α-syn caused significant nigrostriatal degeneration including appearance of dystrophic striatal neurites, loss of nigral dopaminergic (DA) neurons and dissolving nigral neuron bodies in a time-dependent manner. In addition, the α-syn overexpressed mice also developed significant deficits in motor function at 12 weeks when the loss of DA neurons exceeded a threshold of 50%. To investigate the sensitivity to neurotoxins in mice overexpressing α-syn, we performed an MPTP treatment with the subacute regimen 8 weeks after rAAV injection. The impact of the combined genetic and environmental insults on DA neuronal loss, striatal dopamine depletion, dopamine turnover and motor dysfunction was markedly greater than that of either alone. Moreover, we observed increased phosphorylation (S129), accumulation and nuclear distribution of α-syn after the combined insults. In summary, these results reveal that the overexpressed α-syn induces progressive nigrostriatal degeneration and increases the susceptibility of DA neurons to MPTP. Therefore, the targeted overexpression of α-syn and the combination with environmental toxins may provide valuable models for understanding PD pathogenesis and developing related therapies.

D-Ala2-GIP-glu-PAL is neuroprotective in a chronic Parkinson's disease mouse model and increases BNDF expression while reducing neuroinflammation and lipid peroxidation

“…Transgenic α-synuclein overexpressing mice develop deficits in motor function at 12 weeks when the loss of DA neurons exceeds a threshold of around 50%. The overexpression of α-synuclein induced progressive nigrostriatal degeneration and increased the susceptibility of DA neurons to MPTP (Song et al, 2015). The results of immunohistochemistry showed that in normal mice, detectable levels of α-synuclein protein were seen in all brain regions studied and especially in the ventral midbrain.…”

How can rAAV‐α‐synuclein and the fibril α‐synuclein models advance our understanding of Parkinson's disease?

“…; Song et al . ). It is important to note that rAAV‐ α‐synuclein is typically unilaterally injected because tests of asymmetric motor behavior can be easily quantified.…”

“…The vast majority of studies utilize the rAAV-a-synuclein in rats, but the model has been adopted in mice which provide more options for availability of knockout and transgenic mice to test whether certain genes protect and enhance neurodegeneration. However, the phenotypes in the mice are variable and further characterization of rAAV-a-synuclein in mice is recommended (St Martin et al 2007;Theodore et al 2008;Ulusoy et al 2012;Harms et al 2013;Oliveras-Salva et al 2013;Song et al 2015). To produce the virus, typically HEK293 cells are transfected with 2-3 plasmids: a transfer plasmid encoding human a-synuclein flanked by rAAV2 inverted terminal repeats which are necessary for replication and packaging; a plasmid which encodes the replication proteins from rAAV2 and capsid proteins a from a particular serotype (1, 2, 5, 7, or 8); and a plasmid that encodes helper factors necessary for a productive infection (also see (Ulusoy et al 2008) for review).…”

“…In general, rAAV-a-synuclein expression leads to a progressive loss of dopaminergic neurons in the SNpc, and loss of terminals in the striatum, similar to PD brains, although the extent of neurodegeneration in the rAAV-asynuclein model is variable (Table 1). Some studies show that rAAV-a-synuclein expression in the SNpc causes defects in behavioral tests, such as increased amphetamineor apomorphine-induced rotations, increased forepaw asymmetry in cylinder tests, and decreased number of steps in the adjusting steps test (Kirik et al 2002;Ulusoy et al 2010a;Decressac et al 2011Decressac et al , 2012bDecressac et al ,c, 2013Koprich et al 2011;Gaugler et al 2012;Gombash et al 2013;Oliveras-Salva et al 2013;Bourdenx et al 2015;Caudal et al 2015;Daniel et al 2015;Salganik et al 2015;Song et al 2015). It is important to note that rAAV-a-synuclein is typically unilaterally injected because tests of asymmetric motor behavior can be easily quantified.…”

Tipping Points and Endogenous Determinants of Nigrostriatal Degeneration by MPTP

“…Thus, findings in the MPTP model are consistent with the possibility that toxic exposures contribute to PD pathogenesis by modifying ASYN proteostasis. Accordingly, reduced ASYN expression led to resistance to MPTP and rotenone [93,94], whereas overexpression resulted in sensitization [95]. Notably, ASYN level alone cannot explain the relative sensitivities of various catecholaminergic neurons; several brain areas that are highly resistant to MPTP express ASYN at similar levels as nigrostriatal neurons, and DA neurons of the (MPP + -resistant) VTA express higher levels of ASYN than do (MPP + -sensitive) noradrenergic neurons of the locus coeruleus [96].…”