In rivers, riverbeds are considered to have dual properties as a short-term sink and a source of further mobilization for microplastics. To better understand the sources, storage, and fate of microplastics in river systems, this study quantified the formation of microplastic hotspots in riverbeds and seasonal variations in microplastic inventories in riverbeds, especially for small-sized microplastics (<330 µm), with a fluorescence-based protocol. This study provides first-hand measured evidence for the sequestration of microplastics in the riverbed under low-flow conditions and its export from the riverbed under high-flow conditions. The results show that riverbeds in urban areas are still hotspots for microplastic pollution and that high inputs of urban microplastics control microplastic load in its downstream areas. Seasonal rainfall exported 34.86 % (equivalent to 4.34 × 10 11 items/8.57 t) of microplastic pollution from the riverbed, and its removal capacity may be related to the rainfall intensity. Wider riverbeds are conducive to the formation of microplastic hotspots due to the flow slow down. Most importantly, rainfall-driven scouring of the riverbed can enhance the pollution of small-sized microplastics in the riverbed, especially the smallest-size microplastics (<100 µm). Therefore, this study not only contributes reliable information about the sequestration and export of microplastics in the riverbed, but also provides a possible mechanism to explain the lack of small-sized microplastics (<330 µm) in the ocean.
SREBP1 (sterol regulatory element binding protein 1) plays an important role in maintaining lipid balance in mammals. Previous research showed that androgen significantly regulated the secretion of lipid-rich musky odor substances from the scented glands of muskrats. Our investigation provides insights into the pathways through which androgen induces SREBP1 expression and affects seasonal changes in triglyceride synthesis in muskrat scented glands. During the breeding season, pronounced Oil Red O staining was confirmed in the scented glandular tissues, and transmission electron microscopy displayed a marked increase in lipid droplets. The scented glands exhibited a higher triglyceride content, which coincided with augmented circulating testosterone, dihydrotestosterone, as well as increased scented glandular testosterone and dihydrotestosterone concentrations. Transcriptomic, proteomic and metabolomic analyses of the scented glands revealed associations with fatty acid biosynthesis, while plasma metabolomic data indicated an increase in metabolites serving as precursors during the breeding season for de novo fatty acid synthesis. Expression levels of genes and proteins related to lipid synthesis and lipid droplets, as well as FASN activity, were significantly elevated in the breeding season. Immunolocalization studies demonstrated the presence of these proteins in glandular and epithelial cells in both seasons. Moreover, in vitro treatment of the scented glandular cells with testosterone resulted in elevated expressions of triglyceride synthesis-related genes. These findings suggested that androgen modulates triglyceride synthesis to enhance lipid droplet formation via the AR-SREBP1-FASN Axis in the scented glands of muskrats, thereby shedding light on the hormonal regulation of lipid metabolism in seasonal breeders.
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