Modified β-glucans promote saponin and fucosylated chondroitin sulfate biosynthesis in sea cucumber Apostichopus japonicus in association with activation of the TLR-MAPK signaling pathway.

Sea cucumber is a high-value marine food that contains characteristic bioactive components, notably saponins and polysaccharides. Strategies promoting their biosynthesis to improve sea cucumber quality are vital for the industry. The impacts of three dietary yeast β-glucans (native β-glucan, ultrasonic-degraded low-molecular-weight β-glucan, and carboxymethylated β-glucan) on the biosynthesis of saponin and fucosylated chondroitin sulfate (FCS) in A. japonicus and the underlying mechanism were investigated. A 28-day feeding trial showed that modified β-glucans improved growth performance and immune responses, as well as elevated the body wall accumulation of saponin and FCS. Mechanistically, modified β-glucans up-regulated the genes involved in the saponin and FCS biosynthesis, including the core mevalonate pathway (hmgcr, mvk, mvd), downstream post-squalene modifications (sqle), and chondroitin backbone biosynthesis (xylt1, chsy1, chpf). Moreover, these β-glucans up-regulated the TLR-MAPK signaling axis genes (tlr, myd88, irak4, traf6, tak1, mkk1, erk, p38) and increased p38/ERK protein abundance and phosphorylation levels. In vivo MAPK inhibition attenuated β-glucan-induced saponin and FCS accumulation and the expression of hmgcr, sqle, xylt1, and chsy1. Our findings reveal that modified β-glucans enhance A. japonicus quality by promoting saponin and FCS biosynthesis in association with the TLR-MAPK signaling axis-mediated immune-metabolic crosstalk.