Altered Capicua expression drives regional Purkinje neuron vulnerability through ion channel gene dysregulation in Spinocerebellar ataxia type 1

Abstract

Selective neuronal vulnerability in neurodegenerative disease is poorly understood. Using the ATXN1[82Q] model of spinocerebellar ataxia type 1 (SCA1), we explored the hypothesis that regional differences in Purkinje neuron degeneration could provide novel insights into selective vulnerability. ATXN1[82Q] Purkinje neurons from the anterior cerebellum were found to degenerate earlier than those from the nodular zone, and this early degeneration was associated with selective dysregulation of ion channel transcripts and altered Purkinje neuron spiking. Efforts to understand the basis for selective dysregulation of channel transcripts revealed modestly increased expression of the ATXN1 corepressor Capicua (Cic) in anterior cerebellar Purkinje neurons. Importantly, lentiviral overexpression of Cic in the nodular zone accelerated both aberrant Purkinje neuron spiking and neurodegeneration. These findings reinforce the central role for Cic in SCA1 cerebellar pathophysiology and suggest that only modest reductions in Cic are needed to have profound therapeutic impact in SCA1. Figure 3. Regionally-dysregulated ion channel genes form a functional module critical for Purkinje neuron pacemaking (A) The distribution of regularly firing, irregularly firing, and non-firing cells was recorded for Purkinje neurons in the anterior cerebellum and nodular zone. (B) Representative trace from a tonic firing wild-type and non-firing ATXN1[82Q] Purkinje neuron in the anterior cerebellum at P35. (C) Representative trace from wild-type and ATXN1[82Q] Purkinje neurons in the nodular zone at P35. (D) Quantification of the afterhyperpolarization (AHP) and ISI of wild-type and ATXN1[82Q] Purkinje neurons in the anterior cerebellum at P35. (E) Similar to (D), quantification of the AHP and ISI of wild-type and ATXN1[82Q] Purkinje neurons in the nodular zone at P35. (F) Representative traces of wild-type Purkinje neurons in the anterior cerebellum at P35. Traces are shown at baseline (left), after perfusion of 200 nM iberiotoxin (middle), and after perfusion of 200 nM iberiotoxin + 4 µM mibefradil. (G and H) Summary distribution of regularly firing, irregularly firing, and non-firing Purkinje neurons before and after perfusion of 200 nM iberiotoxin (G) and after 200 nM iberiotoxin + 4 µM mibefradil (H). ** denotes p<0.01; *** denotes p<0.001; ns denotes p>0.05; Chi-square test (A, G, H); two-way repeated measures ANOVA with Holm-Sidak correction for multiple comparisons (D, E). Supplementary Figure 3. Spontaneous Purkinje neuron firing in ATXN1[82Q] mice and wild-type controls Patch clamp electrophysiology in acute cerebellar slices from ATXN1[82Q] mice and wild-type controls was performed at P35. (A) Firing frequency in anterior cerebellum and the nodular zone is shown. (B) Coefficient of variation (CV) of the interspike interval (ISI) is shown for Purkinje neurons in the anterior cerebellum and nodular zone. ns denotes p>0.05; two-tailed Student's t-test.

Aberrant Cerebellar Circuitry in the Spinocerebellar Ataxias

“…Furthermore, this neuronal population was also found to highly express CIC ( Chopra et al, 2020 ). Collectively, the findings from Chopra et al (2020) provide the first experimental evidence associating altered Purkinje cell physiology and impaired calcium homeostasis with transcriptional dysregulation in SCA1, a consequence of altered ATXN1-CIC interactions.…”

“…More recently, Chopra et al (2020) expanded on the findings of Rousseaux et al (2018) , highlighting regional differences in Purkinje cell degeneration and correlating these changes with regional patterns of transcriptional dysregulation. Interestingly, several ion channel genes, such as KCNMA1 , CACNA1G , TRPC3 , ITPR1 , and GRM1 , were found to be downregulated in anterior Purkinje cells isolated from SCA1 mice ( Chopra et al, 2020 ). Furthermore, this neuronal population was also found to highly express CIC ( Chopra et al, 2020 ).…”

“…Interestingly, several ion channel genes, such as KCNMA1 , CACNA1G , TRPC3 , ITPR1 , and GRM1 , were found to be downregulated in anterior Purkinje cells isolated from SCA1 mice ( Chopra et al, 2020 ). Furthermore, this neuronal population was also found to highly express CIC ( Chopra et al, 2020 ). Collectively, the findings from Chopra et al (2020) provide the first experimental evidence associating altered Purkinje cell physiology and impaired calcium homeostasis with transcriptional dysregulation in SCA1, a consequence of altered ATXN1-CIC interactions.…”

“…However, many SCAs that yield toxic enhancements in gene function, also yield concomitant loss of interacting partner function. For example, mutant ATXN1 is generally considered to possess enhanced function, however functionality of the transcriptional co-repressor CIC is decreased when interacting with mutant ATXN1 ( Rousseaux et al, 2018 ), resulting in downregulation of many key ion channel channels that are critical for maintaining Purkinje cell function ( Chopra et al, 2020 ). Hence, it can be argued that SCA1 confers concomitant gain and loss of function effects ( Lam et al, 2006 ).…”

A Chlorzoxazone‐Baclofen Combination Improves Cerebellar Impairment in Spinocerebellar Ataxia Type 1

“…While the six genetic causes of CAG triplet repeat-associated SCA are unrelated, recent work illustrates that shared underlying mechanisms of disease may contribute to neuronal dysfunction across these disorders [2][3][4][5]. SCA1, one of the most common and well-studied of the SCAs, is caused by a glutamine-encoding CAG triplet expansion in the ATXN1 gene.…”