Identification and analysis of lipid droplet-related proteome in the adipose tissue of grass carp (Ctenopharyngodon idella) under fed and starved conditions

Understanding interactions between four main fishy compounds and grass carp myofibrillar proteins using the SPME-GC-MS, multiple spectroscopy, and molecular docking

The interaction mechanism between four fishy compounds and myofibrillar proteins of grass crap was explored using solid phase microextraction-gas chromatography-mass spectrometry, multispectroscopy, and molecular docking. The result showed that the binding abilities of myofibrillar protein for the fishy compounds decreased in the order of decanal, octanal, hexanal, and 1-octen-3-ol. The interaction between myofibrillar proteins and four fishy compounds affected the aromatic amino acid residue microenvironment. The predominant binding force of myofibrillar proteins to the three aldehydes was hydrophobic, while those to 1-octen-3-ol were hydrogen bonds and van der Waals forces, and binding sites of these compounds occurred near tryptophan and tyrosine. A significant reduction in α-helical content and surface hydrophobicity in grass carp myofibrillar protein upon interaction with the four fishy compounds. Molecular docking confirmed that the different functional groups and chain lengths of the fishy components resulted in different binding sites and binding free energies with grass carp protein.

Proteome of granulosa cells lipid droplets reveals mechanisms regulating lipid metabolism at hierarchical and pre-hierarchical follicle in goose

Avian hierarchical follicles are formed by selection and dominance of pre-hierarchical follicles, and lipid metabolism plays a pivotal role in this process. The amount of lipid in goose follicular granulosa cells increases with the increase of culture time, and the neutral lipid in the cells is stored in the form of lipid droplets (LDs). LD-associated proteins (LDAPs) collaborate with LDs to regulate intracellular lipid homeostasis, which subsequently influences avian follicle development. The mechanism by which LDAPs regulate lipid metabolism in goose granulosa cells at different developmental stages is unclear. Therefore, using BODIPY staining, we found that at five time points during in vitro culture, the LD content in hierarchical granulosa cells was significantly higher than that in pre-hierarchical granulosa cells in this study (p < 0.001). Next, we identified LDAPs in both hierarchical and pre-hierarchical granulosa cells, and screened out 1,180, 922, 907, 663, and 1,313 differentially expressed proteins (DEPs) at the respective time points. Subsequently, by performing Clusters of Orthologous Groups (COGs) classification on the DEPs, we identified a large number of proteins related to lipid transport and metabolism. Following this, the potential functions of these DEPs were investigated through Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) functional enrichment analysis. Finally, the important pathway of fatty acid degradation and the key protein ACSL3 were screened out using Short Time-series Expression Miner (STEM) and Protein-Protein Interaction (PPI) analysis methods. It is hypothesized that ACSL3 may potentially modulate lipid metabolism through the fatty acid degradation pathway, thereby contributing to the difference in lipid content between hierarchical and pre-hierarchical granulosa cells. These findings will provide a theoretical foundation for further studies on the role of LDs and LDAPs in avian follicle development.

Protein Sample Preparation for Bottom-Up, Label-Free Quantitative Proteomics of Adipose Tissue

Adipose tissue (AT) is a complex, multifunctional endocrine organ that plays a significant role in animal evolution and human disease. Profiling of the proteome, or the set of proteins produced by a cell or tissue at a given time, can be used to explore the myriad functions of adipose tissue and understand its role in health and disease. The main challenges of adipose tissue proteomics include the high lipid and low protein content of the tissue and association of many proteins with lipid droplets. Here, we present a protocol for gel-free, label-free, bottom-up, relative quantitative proteomics of adipose tissue based on findings from the literature and our laboratory that yields reproducible protein and peptide identification rates while minimizing cost and processing time. This approach involves tissue homogenization, protein precipitation from homogenates, solubilization and denaturation of proteins in a buffer containing 5% sodium deoxycholate (SDC, an acid-insoluble detergent) and 5 mM tris(2-carboxyethyl)phosphine (TCEP, a reducing agent), alkylation with chloroacetamide, and in-solution tandem digestion with trypsin and Lys-C enzymes in the presence of 1% SDC. Acidification of peptides efficiently removes SDC prior to desalting and mass spectrometry. This method has been used successfully in our laboratory by both experienced researchers and those with limited technical backgrounds, including high school, undergraduate, and graduate students. We have identified >1500 proteins in adipose tissue of non-model mammals (e.g., blubber of marine mammals) spanning a dynamic range of 105 using this approach, including proteins of interest for comparative physiology such as adipokines, metabolic and antioxidant enzymes, lipid droplet proteins, metabolite transporters, and mitochondrial proteins, among others.

Isolation and proteomic study of fish liver lipid droplets

Lipid droplets (LDs) are a neutral lipid storage organelle that is conserved in almost all species. Excessive storage of neutral lipids in LDs is directly associated with many metabolic syndromes. Zebrafish is a better model animal for the study of LD biology due to its transparent embryonic stage compared to other organisms. However, the study of LDs in fish has been difficult due to the lack of specific LD marker proteins and the limitation of purification technology. In this paper, the purification and proteomic analysis of liver LDs of fish including zebrafish and Carassius auratus were performed for the first time. 259 and 267 proteins were identified respectively. Besides most of the identified proteins were reported in previous LD proteomes of mammals, indicating the similarity between mammal and fish LDs. We also identified many unique proteins of liver LDs in fish that are involved in the regulation of LD dynamics. Through morphological and biochemical analysis, we found that the marker protein Plin2 of zebrafish LD was located on LDs in Huh7 cells. These results will facilitate further study of LDs in fish and liver metabolic diseases using fish as a model animal.

Prostaglandin 2α Promotes Autophagy and Mitochondrial Energy Production in Fish Hepatocytes

Fatty liver, characterized by excessive lipid droplet (LD) accumulation in hepatocytes, is a common physiological condition in humans and aquaculture species. Lipid mobilization is an important strategy for modulating the number and size of cellular LDs. Cyclooxygenase (COX)-mediated arachidonic acid derivatives are known to improve lipid catabolism in fish; however, the specific derivatives remain unknown. In the present study, we showed that serum starvation induced LD degradation via autophagy, lipolysis, and mitochondrial energy production in zebrafish hepatocytes, accompanied by activation of the COX pathway. The cellular concentration of PGF2α, but not other prostaglandins, was significantly increased. Administration of a COX inhibitor or interference with PGF2α synthase abolished serum deprivation-induced LD suppression, LD-lysosome colocalization, and expression of autophagic genes. Additionally, exogenous PGF2α suppressed the accumulation of LDs, promoted the accumulation of lysosomes with LD and the autophagy marker protein LC3A/B, and augmented the expression of autophagic genes. Moreover, PGF2α enhanced mitochondrial accumulation and ATP production, and increased the transcript levels of β-oxidation- and mitochondrial respiratory chain-related genes. Collectively, these findings demonstrate that the COX pathway is implicated in lipid degradation induced by energy deprivation, and that PGF2α is a key molecule triggering autophagy, lipolysis, and mitochondrial development in zebrafish hepatocytes.

Key Factors Affecting the Flesh Flavor Quality and the Nutritional Value of Grass Carp in Four Culture Modes

Flavor and nutritional value are important qualities of freshwater fish products, but the key factors affecting these quality parameters remain unclear. In this study, four typical aquaculture modes, including the commercial feed treatment (control), faba bean treatment (FBT), grass powder treatment (GPT), and waving water treatment with commercial feed (WWT), were used to explore the regulatory effect of water quality and feed (eaten and uneaten) on the flesh flavor and nutrition in grass carp (Ctenopharyngodon idella), a freshwater fish of the largest global production. During the culture period (90 days), water quality parameters of the four modes were measured every 15 days, and the flavor quality was evaluated by volatile flavor compounds detection and electronic nose analyzer. Flesh crude protein, crude fat, free fatty acid and free amino acid profiles were also determined. The results showed that, in the late period, the FBT mode had the poorest water quality with highest concentrations of nitrite and nitrate, while the GPT mode has the best water quality among the four modes. Most flesh flavor compounds found in the flesh of the control, GPT and WWT modes were pleasant. In the FBT mode with the poorest water quality, on the other hand, we found lower flavor quality (higher contribution of fishy compounds), higher water content, and lower contents of crude protein, crude fat, free fatty acids and free amino acids, compared to the other three modes. Correlation analyses showed that nitrite and nitrate are probably key water quality factors affecting the flavor quality and nutritional values besides eaten feed and uneaten feed factors. This study can serve as an important reference for ecological regulation and feeding administration of flesh quality in freshwater aquaculture fish.