Cell culture is a staple of modern biology, and Fetal Bovine Serum (FBS) is an essential component of many cell culture protocols. A specific use for FBS is to supply nutrients to cells and to stimulate their growth. Another role of FBS in cell culture research is to represent the complexities and functionality of endogenous biological environments; however, precisely this complexity has long been a potential confounding factor for researchers. For example, cytokines in FBS can lead to the stimulation of cells, thus producing unintended experimental environments. Despite these disadvantages, FBS retains a prominent role in modern cell culture, with estimated sales as high as 700,000 liters per year. Because of its ubiquity in cell culture research, it is critical to investigate how the components of FBS may be influencing experiments and downstream analysis.
Variability and uncertainty in the composition of FBS is especially problematic for studies that evaluate cellular secretions. For example, to successfully determine the array of RNA secreted by cultured cells, we need to know the extent to which the medium is contaminated by exogenous RNA. Additionally, extracellular RNA (exRNA) is not only found distributed freely throughout the liquid medium, but it is also often found packaged inside of extracellular vesicles (EVs) or lipoprotein complexes. Therefore, in a paper released online yesterday, Wei et al. evaluated how the RNA composition of FBS might be confounding research.
The authors first evaluated exogenous RNA contamination. They grew cultures of a cell type known not to express a particular RNA, then evaluated the presence of that RNA in the culture media. If that RNA was found, its origin was probably the media itself. For example, the authors demonstrated that miR-122, a liver-specific miRNA, is present in media from cultured glioma cells, suggesting that its source is likely FBS itself. They then attempted to deplete RNA from FBS via ultracentrifugation, but despite a 24 hour spin at 100,000g, about 75% of total RNA remained in the supernatant. This result has also been found by researchers attempting to deplete FBS of RNA-containing EVs and emphasizes the difficulty of producing media truly free from contaminating RNA.
These results led the authors to ask whether existing studies have wrongly attributed the presence of exRNA to a particular experimental procedure or cell type, when it should be recognized as a component of the FBS in the cell culture media. To answer this question, the authors first broadly profiled the RNA composition of FBS using RNA sequencing. They determined that between 9% and 22% of FBS RNA mapped to the human genome, depending on the stringency of the mapping algorithm and FBS preparation. They also checked for the presence of bovine-specific RNA in existing human cell culture exRNA datasets, finding levels as high as 17%, with samples from exosomes (a type of EV) containing particularly high levels. Finally, they demonstrated experimentally that bovine-specific transcripts are taken up into cells, interfering not only with exRNA analysis but also with intracellular RNA studies.
Moving forward, a significant remaining issue is deciding how to treat conserved RNA known to be present in both FBS and the cell line under study. Switching from FBS to purely chemically defined media can help with this problem, but it is not possible for all cell types and experimental conditions. Alternatively, a quantitative analysis of the chemical composition of the media might make it possible to estimate which RNAs are secreted by the cells of interest by filtering out known FBS RNAs from the total RNA pool.
This research cautions us to be careful in the design and interpretation of experiments to identify extracellular RNAs that use FBS in culture media. The paper, Fetal Bovine Serum RNA Interferes with the Cell Culture derived Extracellular RNA, released in Scientific Reports yesterday, is authored by Zhiyun Wei, Arsen O. Batagov, David R. F. Carter, and Anna M. Krichevsky.