Microgravity induces morphological and molecular reprogramming in Atlantic Salmon (Salmo salar) kidney cell line.

Microgravity provides cells models that physiologically mimic in vivo tissue organization. However, microgravity effects on fish cell systems remain poorly understood. This study investigated structural, ultrastructural, and transcriptomic adaptations of Atlantic salmon kidney (ASK) cell line cultured as 3D spheroids under microgravity. Light microscopy and confocal imaging revealed that ASK cells formed compact, well-defined spheroids with organized nuclear distribution and extensive membrane networks. Flow cytometry and Transmission Electron microscopy (TEM) confirmed heterogeneous spheroid populations and extensive ultrastructural remodeling. Transcriptomic analysis revealed that a total of 6404 genes were differentially expressed, with 3005 upregulated and 3399 downregulated. Moreover, 41 GO terms and KEGG pathways were globally dysregulated. Upregulated pathways included melanogenesis, lysosome-autophagy-phagosome systems, metabolic homeostasis (PPAR signaling, lipid and glycan metabolism), ECM-receptor interaction, stress-adaptive signaling (MAPK, mTOR, FoxO), and immune surveillance (Toll-receptor signalling, cytokine-cytokine receptor interaction), indicating enhanced survival, membrane remodeling, and immune-like activity. In contrast, downregulated pathways were associated with cell cycle progression, DNA replication and repair, metabolic biosynthesis, and developmental signaling, reflecting reduced proliferation and a shift toward metabolic quiescence. qPCR validation confirmed increased expression of genes linked to melanogenesis, morphogenesis, and PPAR signaling. Notably, this transcriptional profile parallels key functional aspects of salmon kidney functions, particularly head kidney roles in immune surveillance, stress integration, and metabolic homeostasis. These findings show that microgravity drives ASK cells to have tissue-associated structural and molecular features. This suggests potential of 3D microgravity spheroids as a physiologically relevant model for studying fish kidney-associated functions, fish immunology and applications in cellular agriculture.