Neuroprotective effects of synaptic modulation in Huntington's disease R6/2 mice

Abstract

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder in which the neostriatum degenerates early and most severely, with involvement of other brain regions. There is significant evidence that excitotoxicity may play a role in striatal degeneration through altered afferent corticostriatal and nigrostriatal projections that may modulate synaptically released striatal glutamate. Glutamate is a central tenant in provoking excitotoxic cell death in striatal neurons already weakened by the collective molecular events occurring in HD. In addition, transcriptional suppression of trophic factors occurs in human and transgenic mouse models of HD, suggesting that a loss of trophic support might contribute to degeneration. Since anti-glutamate approaches have been effective in improving disease phenotype in HD mice, we examined whether deafferentation of the corticostriatal and nigrostriatal pathways may mitigate striatal stress and neurodegeneration. Both surgical and chemical lesions of the corticostriatal and nigrostriatal pathways, respectively, improved the behavioral, neuropathological, and biochemical phenotype in R6/2 transgenic mice and extended survival. Decortication ameliorated hindlimb clasping, striatal neuron atrophy, and huntingtin-positive aggregates, improved N-acetyl aspartate/creatine levels, reduced oxidative stress, and significantly lowered striatal glutamate levels. In addition, 6-hydroxydopamine lesioned mice showed extended survival along with a significant reduction in striatal glutamate. These results suggest that synaptic stress is likely to contribute to neurodegeneration in HD, whereas transsynaptic trophic influences may not be as salient. Thus, modulation of synaptic influences continues to have therapeutic potential in HD.

Figures

Figure 1.

Effects of cortical, cerebellar, and nigral lesions on R6/2 survival. A, B, Cortical lesions performed at 4 or 6 weeks in R6/2 mice significantly extended survival compared with nonlesioned R6/2 mice by 13.4 and 8.25% (4 weeks, χ2 = 15.44, p < 0.01; 6 weeks, χ2 = 16.64, p < 0.01). C, In contrast, cerebellar lesions had no effect on R6/2 survival. D, 6-OHDA lesions of the substantia nigra also significantly extended survival by 21.29% compared with sham-treated R6/2 mice (χ2 = 13.83, p < 0.01).

Figure 2.

Effect of cortical deafferentation on dystonia starting at 6 weeks in R6/2 mice. Cortical lesions significantly improve clasping of the hindlimbs at 8 and 10 weeks compared with nonlesioned R6/2 mice, as determined by Fisher's exact probability test. * p < 0.01.

Figure 3.

The extent of gross morphology of the cortical aspiration lesion in a representative decorticate R6/2 mouse (A–K). The lesion is restricted to the primary motor cortex, M1 and M2. Scale bar, 1 mm.

Figure 4.

Effects of cortical deafferentation on striatal neuron morphology. The normal morphology of wild-type dorsolateral striatum is shown in A. The morphology of the M1/M2 ipsilateral projection field in the dorsolateral striatum in a decorticated R6/2 mouse is shown in B and as outlined in D with surrounding atrophic neurons. This is a region of relative normal neuronal size. The remaining neostriatum shows angular, shrunken neurons characteristically observed in nonlesioned R6/2 mice, as shown in C (see Table 1). Scale bar, 100 μm.

Figure 5.

Effect of cortical deafferentation on huntingtin aggregates in the ipsilateral striatum. Huntingtin aggregates are a prominent neuropathological hallmark observed in the R6/2 mouse and HD patients. Cortical deafferentation attenuates huntingtin aggregates in the M1/M2 projection field of the neostriatum, as indicated by the asterisk (B and Table 1) in contrast to striatal mutant huntingtin aggregates in R6/2 mice (A). Scale bar, 200 μm.

Figure 6.

The effects of 6-OHDA lesions of the substantia nigra on huntingtin aggregation in the striatum. Mutant huntingtin aggregates in the striatum of non-6-OHDA lesioned R6/2 mice (A) is not altered by unilateral 6-OHDA lesions of the substantia nigra in either the ipsilateral or contralateral striatum (B, C). There is, however, a marked reduction of dopamine transporter immunoreactivity in the ipsilateral striatum (D). Scale bar: (in A) A, 100 μm; D, 1 mm.