Midgut BmFABP2/3 link intestinal lipid homeostasis to stress remodeling and systemic traits in Bombyx mori

Fatty acid-binding proteins (FABPs) act as intracellular lipid chaperones that facilitate cytosolic trafficking and metabolic routing of hydrophobic ligands, yet in vivo genetic evidence for their tissue-specific functions in Lepidoptera remains limited. Here, we characterize two highly homologous FABP-like genes, BmFABP2 and BmFABP3, that show strong midgut-enriched expression and reversible transcriptional responses to starvation-refeeding transitions and exogenous fatty acid stimulation. Using a homozygous BmFABP2/3 double-knockout line and a midgut-specific BmFABP2 overexpression line, we demonstrate that this module is closely associated with intestinal lipid homeostasis and cross-tissue phenotypic outputs. Loss of BmFABP2/3 elevates midgut free fatty acid levels and enhances neutral lipid deposition in both the midgut and fat body, accompanied by coordinated transcriptional changes in genes involved in lipid droplet turnover and triacylglycerol metabolism. Integrated transcriptomics and untargeted metabolomics further reveal remodeling of pathways linked to glutathione metabolism, cytochrome P450-mediated detoxification, and lysosome-associated processes, together with reprogramming of antioxidant and detoxification gene networks. Notably, mutants display a stable deepening of larval body pattern pigmentation across instars, coupled with concerted transcriptional shifts in key nodes of catecholamine/melanin metabolism in patterned epidermal regions. Midgut-specific BmFABP2 overexpression shows trends opposite to those observed in the knockout at both molecular and phenotypic levels. Collectively, these results support a genetically tractable link between midgut fatty acid trafficking and metabolic routing, stress-associated reprogramming, and distal pigmentation-related outputs, providing reusable genetic resources and a framework for dissecting intestinal metabolic control of organismal traits.