Accumulation of pathological Ataxin‐7 in the medulla leads to hypoglossal (XII) motor unit pathology

Abstract

SCA7 is an autosomal dominant neurological disorder caused by a deleterious CAG repeat expansion in the coding region of the ataxin‐7 gene on chromosome 3. Patients with infantile SCA‐7 have the largest repeat expansion characterized by progressive loss of coordination, dysarthria, dysphagia, retinal degeneration and respiratory distress. These infants succumb to death due to severe hypotonia, aspiration pneumonia and respiratory failure. Our objective is to comprehensively analyze the underlying mechanisms that result in upper airway pathology which exacerbates respiratory insufficiency in SCA7. To do this, we will use an infantile SCA7 mouse model: SCA7266Q Knock‐in (KI) mouse. SCA7266Q KI mice exhibit progressive respiratory dysfunction along with irregular breathing and prolonged apneic events with a significant loss of hypoglossal (XII) motoneurons in the medulla. The XII motoneurons innervate the tongue to maintain the shape, stiffness and position of the tongue and regulate the upper airway patency. Therefore, we hypothesize that the expression of pathological ataxin‐7 in the medulla causes XII motor unit pathology leading to upper airway pathology. To confirm if the reduced number of motoneurons resulted in hypoglossal nerve pathology, we analyzed the efferent nerve output of the XII nerve of 9‐week‐old WT control and SCA72666Q KI mice during eupnea and hypercapnia (7% CO2). Later we performed post‐mortem histological analysis of the XII respiratory centers in the medulla and XII nerves. Specifically, we used immunohistochemistry to identify the cells that accumulate ataxin‐7 and we performed TUNEL assay to evaluate the extent of neurodegeneration in the XII motor pool. We analyzed the demyelination of the nerves by light microscopy and electron microscopy. To identify the cellular mechanisms involved in the XII pathology, we performed transcriptome analysis by bulk RNA sequencing of the medulla and confirmed the findings with qPCR. Results XII nerve efferent output is blunted at baseline and hypercapnic challenge in SCA7266QKI mice. There is increased TUNEL+ cells in the XII motor pool with intranuclear accumulation of ataxin‐7 in the astrocytes. Further, demyelination is present in the XII nerves in SCA7266QKI mice. RNA sequencing data of the medulla supports these findings and reveals diminished expression of the transcripts regulating the myelination pathway such as Plp1. There is also a significant decrease in activation of astrocytes with attenuated expression of neurotransmitter transporter such as Slc6a9 and Slc710 that are critical for breathing. In conclusion, intranuclear accumulation of ataxin‐7 in SCA7266QKI mice modulates transcription of astrocytes and influences different signaling pathways that result in neurodegeneration, demyelination, and decreased efferent XII nerve output. These findings are clinically relevant and emphasize the importance of neuropathology that results in dysphagia, aspiration pneumonia and respiratory failure in infantile SCA7.