Modification of early behavioural, physiological and neuropathological endpoints by syntaxin-6 knockout in a humanised P301S transgenic model of tauopathy

Genetically mediated increased expression of syntaxin-6, a SNARE protein involved in intracellular protein trafficking, is a proposed risk mechanism for progressive supranuclear palsy and sporadic prion disease. Increased syntaxin-6 protein levels are also causally associated with Alzheimer's disease, suggesting it may have shared roles across multiple neurodegenerative diseases. However, no study has validated its functional role in tauopathies. To validate a role for syntaxin-6 in tauopathy pathogenesis, we knocked out syntaxin-6 in humanised P301S tauopathy mice. Mice underwent longitudinal rotarod testing, gait analysis, frailty and weight assessment, with neuropathological, biochemical and pathological analyses at 3 and 5 months. Stx6+/+;hTauP301S/P301S mice showed motor impairment from 1 month of age, which was partially rescued by syntaxin-6 knockout from months 1 to 4, with additional protection of gait at 5.5 months. Physiologically, syntaxin-6 knockout exerted a protective effect on weight trajectories and measures of frailty. Reduced neurodegeneration in the superficial cortex was observed at 3 months, as well as higher synaptic coverage at 5 months of age, supporting preserved neuropathological measures related to function. We further observed localised increases in tau pathology in the spinal cord and defined brain regions in young Stx6-/-;hTauP301S/P301S mice, despite total tau levels being comparable, in keeping with altered trafficking of pathological tau species with syntaxin-6 knockout. Despite a partial, early phenotypic rescue of functional measures, terminal endpoint comparisons were confounded by a 20% weight loss culling rule, as knockout mice maintained higher absolute weight. Taken together, this study functionally validates a role for syntaxin-6 in tauopathy pathogenesis, with syntaxin-6 knockout resulting in an early protective effect on multiple disease-relevant phenotypes in a humanised tauopathy model.