Hyperexcitable molecular layer interneurons drive cerebellar circuit dysfunction in spinocerebellar ataxia type I

Douthwaite, Christopher (2024): Hyperexcitable molecular layer interneurons drive cerebellar circuit dysfunction in spinocerebellar ataxia type I. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)

Abstract

Spinocerebellar ataxia type 1 (Sca1) is an autosomal dominant, neurodegenerative disease, emerging from a CAG repeat expansion mutation in the gene ATXN1. Classical patient symptoms include motor incoordination and abnormal gait, which coincide with the hallmark pathology of atrophy across the cerebellum and brainstem, in particular, degeneration of the cerebellar cortex Purkinje neurons (PN). Given that there are currently no disease modifying treatments for patients, it is imperative to understand the pathogenesis of Sca1. Research has primarily focussed on how the ATXN1 mutation leads to PN dysfunction and ultimately degeneration. However, ATXN1 is ubiquitously expressed, and many cell types other than PNs degenerate, thus are likely also altered in Sca1 (Seidel et al., 2012). Indeed, recent evidence has shown that in the presymptomatic stage of a Sca1 mouse model there is already an increase in synaptic connections from surrounding inhibitory neurons onto PNs (Edamakanti et al., 2018). Despite such findings, early alterations in distinct neuronal populations of the cerebellar cortex, and their interactions as a circuit, are poorly understood. To investigate early cellular and circuit dysfunction in distinct neuronal populations of the cerebellar cortex, I investigated a Sca1 mouse model as symptoms are emerging. Specifically, I utilized in vivo two-photon calcium imaging, simultaneously recording the three primary inhibitory neurons of the cerebellar cortex circuit; PNs, molecular layer interneurons (MLINs) and Golgi cells. In order to unravel alterations in either spontaneous activity or response properties to cerebellum-associated behaviours, we recorded neuronal calcium signals in anesthetised mice and during a range of awake conditions. The most prominent deficits emerged in the MLIN population, which were hyperactive during quiet wakefulness and hyperresponsive to sensorimotor input MLIN dysfunction also appeared to drive a breakdown in the capacity of the cerebellar cortex to encode sensorimotor information. The PN dendrites, which receive extensive input from MLINs, also displayed enhanced calcium signals. To establish the pathophysiological relevance of these findings to Sca1, I used chemogenetic tools to specifically inhibit MLINs. Acute prevention of MLIN hyperactivity in Sca1 mice reduced aberrant PN dendrite calcium signals, restored network encoding, and most importantly, improved motor coordination. Thirty-day chronic inhibition of MLINs induced lasting motor improvements and delayed disease progression. The critical role of hyperactive MLIN in triggering symptoms typical of Sca1, was further corroborated by mimicking the increased MLIN excitability through chemogenetic stimulation of MLINs in healthy mice. Acute stimulation of MLINs disrupted motor function and could drive pathological features in PN dendrites, whilst chronic MLIN stimulation in young healthy mice induced lasting motor impairments and reduction of PN post-synapses reminiscent of Sca1 pathology, detectable over four months after treatment end. These findings together show, for the first time, that aberrant MLIN activity is a clear feature of early Sca1, and can drive neuronal circuit dysfunction. Moreover, our experiments revealed that non-cell autonomous mechanisms are sufficient in driving PN pathology. Crucially, we showed that selectively targeting MLINs in the early stages of the disorder alleviates classical motor symptoms in Sca1 mice, opening up a novel therapeutic avenue.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Spinocerebellar ataxia (SCA), neuronal dysfunction, two-photon calcium imaging, cerebellum, molecular layer interneurons, pathophysiology, chemogenetic modulation
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Graduate School of Systemic Neurosciences (GSN)
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 15. Januar 2024
• 1. Bericht­erstatter:in: Liebscher, Sabine