Hyperactivity of the non-canonical inflammasome in SPG11 and SPG48

Hereditary spastic paraplegia (HSP) denotes a heterogeneous group of neurodegenerative spastic gait disorders. Variants in SPG11 cause the most common autosomal recessive HSP also known as SPG11. The gene product Spatacsin interacts with the adaptor protein complex 5 (AP5). Because neurodegeneration in SPG11 is accompanied by marked neuro-inflammation, we hypothesised that Spatacsin may play a cell-autonomous role in pro-inflammatory cells.

Inflammasome activation was assessed in primary microglia and bone-marrow-derived-macrophages (BMDMs) from wild-type, Spg11, and Ap5z1 knockout (KO) mice and monocyte-derived-macrophages (MDMs) from patients with SPG11 mutations. Wild-type and Spg11 KO mice were used to study microglia activation and LPS-induced inflammatory responses in vivo.

We show that microglia activation is more pronounced in pre-symptomatic Spg11 KO compared with control mice following systemic lipopolysaccharide (LPS) challenge. Our subsequent studies demonstrate that the activation of the non-canonical inflammasome results in a stronger inflammatory response in primary microglia and BMDMs from Spg11 KO mice, while the canonical pathway is unaffected. These findings are also observed in MDMs isolated from patients carrying loss-of-function SPG11 mutations. In vivo, LPS triggers a much stronger inflammatory response and leads to drastically increased lethality in Spg11 KO mice. Mass spectrometry of activated BMDMs unveils a massive downregulation of AP5 subunits upon disruption of Spg11. Notably, the disruption of its ζ-subunit Ap5z1, which is associated with SPG48, also sensitises the non-canonical inflammasome.

Our findings suggest that a hyper-reactivity of the non-canonical inflammasome in innate immune cells contributes to neuro-inflammation in SPG11 and SPG48.

Please see Acknowledgements.