| Spinocerebellar ataxia type 1 (SCA1) is a dominantly inherited neurodegenerative disorder caused by the expansion of a CAG trinucleotide repeat tract in the coding region of ATAXIN-1 (ATXN1). The discovery that Purkinje cell (PC) dysfunction is the root cause of the onset of behavioral problems in SCA1, rather than cell death, led to a series of experiments that provided support for altered synaptic signaling. Thus, the goals of this thesis were to test the hypotheses that the excitatory cerebellar cortical circuitry both functionally and morphologically is altered in affected ATXN1 mice expressing human ATXN1 in a PC-specific manner.;Flavoprotein autofluorescence optical imaging, conventional extracellular field potential (FP) recordings, anterograde labeling, and immunofluorescence staining were utilized to characterize the two major afferent projections to PCs, parallel fibers (PFs) and climbing fibers (CFs). By all measures, significant decrements in PF-PC synaptic transmission occurred between 28-40 weeks of age in ATXN1[82Q] mice, when both the optical response and post-synaptic PC component of the FP declined. No alterations in PF-PC synaptic transmission were detected in any ATXN1 genotype in which PC pathology does not progress to cell death. In contrast, activation of CFs by direct stimulation of their neuronal origin, the contralateral inferior olive (CIO), revealed that all affected ATXN1 mice showed deficits in CF-PC synaptic transmission. Mice expressing pathogenic ATXN1 with an expanded polyQ tract showed the severest deficits in CF-PC synaptic transmission early in disease (i.e. 12 weeks of age). Mice expressing expanded ATXN1[82Q] with a phosphomimetic Ser-to-Asp mutation at residue 776 (ATXN1[82Q]-D776) were functionally indistinguishable from ATXN1[82Q] animals. At the same age, mice over-expressing non-pathogenic ATXN1[30Q] (i.e., ATXN1[30Q]/ATXN1[30Q] animals) and pathogenic ATXN1[30Q]-D776 showed comparable reductions in CF-PC synaptic transmission, though to a lesser degree than in ATXN1[82Q]-expressing and ATXN1[82Q]-D776 mice. Morphological studies corroborated the functional results and revealed mislocalized CF-PC synapses in affected ATXN1 animals. Interestingly, ATXN1[30Q]-D776 mice showed the greatest failure of CF terminals to properly translocate from the PC cell body up the dendritic arbor.;These data establish that over-expressing mutant ATXN1 has a deleterious effect specifically on CF-PC synaptic transmission by a mid-stage disease time point, but has little, if any, effect on PF-PC synaptic transmission until late disease stages. Over-expressing non-pathogenic ATXN1[30Q] also has a dose-dependent effect on CF-PC synaptic transmission, albeit to a lesser extent than ATXN1[82Q]. Furthermore, in agreement with previous studies, ATXN1 must enter the nucleus to impact CF-PC synaptic transmission and critically impacts PC physiology during development. Finally, these results establish a discrete functional deficit in ATXN1 mice that likely underlies the onset of movement abnormalities in mice and can be utilized as a quantifiable parameter in further work. |
