BACKGROUND: Obstructive sleep apnea (OSA) independently worsens atherosclerosis. Yet, the mechanisms through which OSA or its characteristic components, intermittent hypoxia and hypercapnia (IHC), increase atherosclerosis risk remain unclear. In humans with OSA, there is an increased abundance of gut bacteria associated with atherosclerosis. IHC alters gut microbiome dynamics in murine atherosclerosis models, which could affect host cardiovascular physiology through bile acids. Furthermore, recently discovered, gut microbiome-synthesized, novel bile acids that are potent agonists of bile acid signaling pathways can affect physiological processes, including atherosclerosis. Secondary bile acids, produced in the gut by the bacterial enzyme bile salt hydrolase (BSH), can act as signaling molecules to facilitate crosstalk between the gut microbiome and host. We hypothesized that IHC-associated atherosclerosis is mediated by microbial bile acid biotransformations.

METHODS: We manipulated the gut microbiome function of Ldlr-/- mice under IHC conditions by overexpressing BSH using engineered native bacteria. Atherogenic diet-fed Ldlr-/- mice, gavaged with engineered bacteria with BSH (BSH+) or without (control, BSH-), were subjected to either room-air (Air, control) or IHC conditions. We assessed host transcriptome changes during the light (ZT3) and dark (ZT13) phases after 6 weeks and atherosclerotic lesion formation, fecal microbiome and metabolome after 12 weeks post-exposure.

RESULTS: Mice under Air/BSH+ conditions had increased atherosclerotic lesions and were similar to those in the IHC/BSH- group, compared to controls. Furthermore, lesion extent when both BSH and IHC were present (i.e. IHC/BSH+) was comparable to levels found in either IHC or BSH alone, implying that luminal BSH overexpression may mimic IHC to promote atherosclerosis. Differential abundance analysis of fecal microbiome and metabolome showed a significant overlap between IHC/BSH- and Air/BSH+ groups, suggesting that IHC and BSH overexpression similarly impact the gut microbiome and metabolome of Ldlr-/- mice. There were similar changes to some novel bile acids under IHC/BSH- and Air/BSH+ conditions that positively correlated with atherosclerotic lesion extent under both BSH-overexpression and IHC conditions. Further, both IHC/BSH- and Air/BSH+ similarly trigger inflammatory and circadian hepatic pathways in the light and dark phases respectively, suggesting that these conditions have an overlapping effect on the host transcriptome in a phase-dependent manner.

CONCLUSIONS: IHC potentiates atherosclerosis through microbial-BSH bile acid biotransformations, suggesting that bile acid modifications are a mechanistic link between OSA-induced microbiome changes and atherosclerosis. Our study highlights a unique, potential target for therapeutic intervention for OSA-related atherosclerosis.

Gunnar Engström

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