Serum Amyloid A3 Fuels a Feed-Forward Inflammatory Response to the Bacterial Amyloid Curli in the Enteric Nervous System

Abstract

Background & aims: Mounting evidence suggests a role for the gastrointestinal microbiome as a determinant of peripheral immunity and central neurodegeneration, but the local disease mechanisms remain unknown. Given its potential relevance for early diagnosis and therapeutic intervention, we set out to map the pathogenic changes induced by bacterial amyloids in the gastrointestinal tract and its enteric nervous system.

Methods: To examine the early response, we challenged primary murine myenteric networks with curli, the prototypic bacterial amyloid, and performed shotgun RNA sequencing and multiplex enzyme-linked immunosorbent assay. Using enteric neurosphere-derived glial and neuronal cell cultures, as well as in vivo curli injections into the colon wall, we further scrutinized curli-induced pathogenic pathways.

Results: Curli induced a proinflammatory response, with marked up-regulation of serum amyloid A3 (Saa3) and the secretion of several cytokines. This proinflammatory state was induced primarily in enteric glia, was accompanied by increased levels of DNA damage and replication, and triggered the influx of immune cells in vivo. The addition of recombinant SAA3 was sufficient to recapitulate this specific proinflammatory phenotype while Saa3 knock-out attenuated curli-induced DNA damage and replication. Similar to curli, recombinant SAA3 caused a strong up-regulation of Saa3 transcripts, indicating a feedforward loop. Colonization of curli-producing Salmonella and dextran sulfate sodium-induced colitis caused a significant increase in Saa3 transcripts, indicating a central role for SAA3 in enteric dysfunction. Inhibition of dual leucine zipper kinase, an upstream regulator of the c-Jun N-terminal kinase pathway responsible for SAA3 production, attenuated curli- and SAA3-induced Saa3 up-regulation, DNA damage, and replication in enteric glia.

Conclusions: Our results position SAA3 as an important mediator of gastrointestinal vulnerability toward bacterial-derived amyloids and demonstrate the potential of dual leucine zipper kinase inhibition to dampen enteric pathology.

Keywords: Curli; Dual Leucine Zipper Kinase; Enteric Nervous System; Microbiome–Gut–Brain Axis; Serum Amyloid A3.