Elevated mutant dynorphin A causes Purkinje cell loss and motor dysfunction in spinocerebellar ataxia type 23

Abstract

Spinocerebellar ataxia type 23 is caused by mutations in PDYN, which encodes the opioid neuropeptide precursor protein, prodynorphin. Prodynorphin is processed into the opioid peptides, α-neoendorphin, and dynorphins A and B, that normally exhibit opioid-receptor mediated actions in pain signalling and addiction. Dynorphin A is likely a mutational hotspot for spinocerebellar ataxia type 23 mutations, and in vitro data suggested that dynorphin A mutations lead to persistently elevated mutant peptide levels that are cytotoxic and may thus play a crucial role in the pathogenesis of spinocerebellar ataxia type 23. To further test this and study spinocerebellar ataxia type 23 in more detail, we generated a mouse carrying the spinocerebellar ataxia type 23 mutation R212W in PDYN. Analysis of peptide levels using a radioimmunoassay shows that these PDYN(R212W) mice display markedly elevated levels of mutant dynorphin A, which are associated with climber fibre retraction and Purkinje cell loss, visualized with immunohistochemical stainings. The PDYN(R212W) mice reproduced many of the clinical features of spinocerebellar ataxia type 23, with gait deficits starting at 3 months of age revealed by footprint pattern analysis, and progressive loss of motor coordination and balance at the age of 12 months demonstrated by declining performances on the accelerating Rotarod. The pathologically elevated mutant dynorphin A levels in the cerebellum coincided with transcriptionally dysregulated ionotropic and metabotropic glutamate receptors and glutamate transporters, and altered neuronal excitability. In conclusion, the PDYN(R212W) mouse is the first animal model of spinocerebellar ataxia type 23 and our work indicates that the elevated mutant dynorphin A peptide levels are likely responsible for the initiation and progression of the disease, affecting glutamatergic signalling, neuronal excitability, and motor performance. Our novel mouse model defines a critical role for opioid neuropeptides in spinocerebellar ataxia, and suggests that restoring the elevated mutant neuropeptide levels can be explored as a therapeutic intervention.

TTBK2 and primary cilia are essential for the connectivity and survival of cerebellar Purkinje neurons

“…Climbing fibers extend from neurons of the Inferior Olivary Nuclei (ION) in the medulla of the brain stem (Purves et al, 2001). These connections are essential for PC functional output, and the dysfunction or loss of these connections, particularly the VGLUT2+ synapses from the climbing fibers, has been shown in various mouse models of SCA to be linked to pathology and disease progression (Duvick et al, 2010;Ebner et al, 2013;Furrer et al, 2013;Smeets and Verbeek, 2016;Smeets et al, 2015).…”

Opioid precursor protein isoform is targeted to the cell nuclei in the human brain

“…A functional role of ΔSP-PDYN should be elucidated, and its molecular partners and targets identified. This is especially important in the light of the recent identification of dominant pathogenic PDYN missense mutations that cause profound neurodegeneration in the human brain [23, 32–34]. These mutations are clustered in the opioid domain overlapping with NLS sequence, and may affect the biogenesis and trafficking of this protein that in turn may contribute to neuropathological changes.…”

Gait abnormalities, ADHD, and environmental exposure to nitrous oxide

“…KOR-mediated abnormal gait in ADHD is supported with the evidence that prodynorphin mutations in mice lead to cytotoxic levels of dynorphin A (DYN A) and contribute to abnormal gait profiles and gradual loss of motor coordination (Smeets et al, 2015). While elevated dynorphin levels may contribute to ataxia and altered gait profiles, constitutive activity of the KOR receptor in rat brain has been recently shown to undergo maturational alterations (Sirohi and Walker, 2015).…”