Altered metabotropic glutamate receptor activity in the early stages of spinocerebellar ataxia type 1.

Abstract

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant, progressive neurodegenerative motor disorder with no known cure or palliative treatment. SCA1 results from a CAG trinucleotide expansion of the ataxin-1 gene. Expression of this mutant ataxin-1 causes Purkinje neuron (PN) and spinal cord degeneration resulting in severe motor impairments although the underlying mechanisms are still a mystery. Nevertheless understanding the cellular consequences of mutant ataxin-1 expression in PNs will aid in the development of a targeted disease treatment. This study aimed to investigate the early stages of SCA1 in order to identify physiological changes that contribute to disease progression. To achieve this aim we used a mouse model of SCA1 that conditionally expresses a mutant ataxin-1 gene with 82 CAG repeats (labelled 82Q mice) specifically in cerebellar PNs. Previous studies showed that expression of mutant ataxin-1 during post-natal development hinders PN maturation and so makes it difficult to identify early mechanistic consequences of mutant ataxin-1 expression. To avoid these complications we repressed mutant ataxin-1 expression during postnatal development (0-6 weeks) and then allowed it to be expressed for a further 6 weeks, we called these mice 82Q off/on. Motor testing revealed that 82Q mice exhibit cerebellar ataxia at 6 (early) and 12 (mid-stage) weeks old compared with age matched WT controls. 12 week old 82Q off/on mice exhibited normal motor behaviour (pre-symptomatic). PNs morphology was also examined using Sholl analysis and, indirectly, using electrophysiological measurements (input resistance and capacitance). PNs from 82Q mice showed signs of degeneration at both 6 and 12 weeks of age however PNs from 82Q off/on mice did not. Previous array analysis from 82Q SCA1 mouse cerebellum showed alterations to important components of glutamate signalling pathways, including metabotropic glutamate receptor 1 (mGluR1) and calcium signalling. Using electrophysiology and live calcium imaging approaches we identified prolonged PF mediated mGluR1 dependent currents in both 82Q (early and mid-stage) and 82Q off/on (pre-symptomatic) SCA1 mice that were in part driven by the loss of the glutamate transporter EAAT4. Furthermore, we observed prolonged synaptic mGluR1 mediated calcium responses specifically in the dendrites of 82Q PNs from mid-stage mice and slower overall calcium handling in the 82Q PN dendrites even after mGluR1 blockade. To determine the in vivo relevance of the prolonged mGluR1 signalling in PNs from SCA1 mice we administered the potent, activity dependent mGluR1 negative allosteric modulator (NAM) JNJ16259685 to specifically reduce the overactive mGluR1 signalling in 82Q. Remarkably JNJ temporarily corrected the behavioural ataxia in both early and mid-stage SCA1 mice. This thesis provides the first preliminary evidence for negative modulation of mGluR1 receptors as a potential treatment for SCA1.