Effect of a pore-forming protein derived from Flammulina velutipes on the Caco-2 intestinal epithelial cell monolayer

Flammutoxin, a Degradation Product of Transepithelial Electrical Resistance-Decreasing Protein, Induces Reactive Oxygen Species and Apoptosis in HepG2 Cells

Proteins from Flammulina filiformis were prepared by sodium chloride extraction and fractionated by ammonium sulfate precipitation with increasing saturation degrees to obtain the protein fractions Ffsp-30, Ffsp-50, Ffsp-70, Ffsp-90, and Ffp-90. Among these protein fractions, Ffsp-50 possessed the most significant cytotoxic effect against three human gastrointestinal cancer cell lines, viz. HT-29, SGC-7901, and HepG2. SDS-PAGE and MALDI-TOF/TOF MS/MS analyses revealed that flammutoxin (FTX) was present as a dominating protein in Ffsp-50, which was further evidenced by HPLC-MS/MS determination. Furthermore, native FTX was purified from Ffsp-50 with a molecular weight of 26.78 kDa, exhibiting notable cytotoxicity against gastrointestinal cancer cell lines. Both Ffsp-50 and FTX exposure could enhance intercellular reactive oxygen species (ROS) generation and induce significant apoptosis in HepG2 cells. FTX was identified to be relatively conserved in basidiomycetes according to phylogenetic analysis, and its expression was highly upregulated in the primordium as well as the pileus of the fruiting body from the elongation and maturation stages, as compared with that in mycelium. Taken together, FTX could remarkably inhibit cell growth and induce ROS and apoptosis in HepG2 cells, potentially participating in the growth and development of the fruiting body. These findings from our investigation provided insight into the antigastrointestinal cancer activity of FTX, which could serve as a biological source of health-promoting and biomedical applications.

Transcriptome data reveal conserved patterns of fruiting body development and response to heat stress in the mushroom-forming fungus Flammulina filiformis

Mushroom-forming fungi are complex multicellular organisms that form the basis of a large industry, yet, our understanding of the mechanisms of mushroom development and its responses to various stresses remains limited. The winter mushroom (Flammulina filiformis) is cultivated at a large commercial scale in East Asia and is a species with a preference for low temperatures. This study investigated fruiting body development in F. filiformis by comparing transcriptomes of 4 developmental stages, and compared the developmental genes to a 200-genome dataset to identify conserved genes involved in fruiting body development, and examined the response of heat sensitive and -resistant strains to heat stress. Our data revealed widely conserved genes involved in primordium development of F. filiformis, many of which originated before the emergence of the Agaricomycetes, indicating co-option for complex multicellularity during evolution. We also revealed several notable fruiting-specific genes, including the genes with conserved stipe-specific expression patterns and the others which related to sexual development, water absorption, basidium formation and sporulation, among others. Comparative analysis revealed that heat stress induced more genes in the heat resistant strain (M1) than in the heat sensitive one (XR). Of particular importance are the hsp70, hsp90 and fes1 genes, which may facilitate the adjustment to heat stress in the early stages of fruiting body development. These data highlighted novel genes involved in complex multicellular development in fungi and aid further studies on gene function and efforts to improve the productivity and heat tolerance in mushroom-forming fungi.

Application of thermal stability difference to remove flammutoxin in fungal immunomodulatory protein, FIP-fve, extract from Flammulina velutipes

Fungal immunomodulatory protein (FIP-fve) is a potential functional food ingredient. However, undesirable component flammutoxin (FTX) would occur in the extracted fraction of FIP-fve. In this paper, an application of heating processing instead of the intensive separation process was employed in fractionation of FIP-fve, meanwhile, exclusion of FTX was reached. Contents of FIP-fve and FTX were monitored by HPLC-UV-ESI-MS. Both FIP-fve and FTX had higher thermal stability in a lower concentration solution. Cold water could effectively extract FIP-fve and FTX from fresh mushroom without acetic acid and disulfide-bond breaking agent β-mercaptoethanol commonly used in biochemical studies. Heating cold water extract contained 580 μg/mL FIP-fve and 452 μg/mL FTX at 60 °C for 5 min could effectively exclude FTX and remain 75% of FIP-fve. Adding 0.1 M trehalose or 20% ethanol did not significantly alter the stability of both proteins. The method developed is an applicable procedure for preparing FIP-fve solution free of FTX.

Combination of on-line desalting and HPLC-UV-ESI-MS for simultaneous detection and identification of FIP-fve and flammutoxin in Flammulina velutipes

A rapid analytical approach, on-line desalting HPLC-UV-ESI-MS method, for the analysis of FIP-fve and flammutoxin (FTX), two important bioactive proteins in the fruiting bodies of Flammulina velutipes, was developed. In this study, a highly efficient desalting method is provided using molecular weight cut-off centrifugal filtration and on-line desalting. Sample preparation followed by an on-line desalting HPLC-UV-ESI-MS system was employed for simultaneous desalting and detection and identification of FIP-fve and FTX. Results indicated that using trifluoroacetic acid as a modifier on a C18 reversed-phase column renders effective separation. ESI-MS revealed that the apparent molecular masses of FIP-fve and FTX were 12,749.1 Da and 21,912.5 Da, respectively. Eleven milligrams of FIP-fve was obtained from 100 g of fresh fruiting bodies, and UV detection was performed at 280 nm using bovine serum albumin as the standard protein. The calibration curve was linear in the concentration range of 0.29–4.69 mg/mL (r² = 0.9999). FTX and a series of degradation products were isolated from F. velutipes using 35% saturated ammonium sulfate on a DEAE cellulose column. The complete identification of FTX and a series of degradation products were carried out by precipitation of various ammonium sulfate concentrations (0–45%, 45–65% and 65–90%), in-gel trypsin digestion, and MS analysis with combined database search. The molecular weights of FTX and a series of degradation products were 29,957.2 Da, 27,480.2 Da, 26,512.5 Da, and 21,912.5 Da.

The Staphylococcus aureus alpha-toxin perturbs the barrier function in Caco-2 epithelial cell monolayers by altering junctional integrity

Increased microvascular permeability is a hallmark of sepsis and septic shock. Intestinal mucosal dysfunction may allow translocation of bacteria and their products, thereby promoting sepsis and inflammation. Although Staphylococcus aureus alpha-toxin significantly contributes to sepsis and perturbs the endothelial barrier function, little is known about possible effects of S. aureus alpha-toxin on human epithelial barrier functions. We hypothesize that S. aureus alpha-toxin in the blood can impair the intestinal epithelial barrier and thereby facilitate the translocation of luminal bacteria into the blood, which may in turn aggravate a septic condition. Here, we showed that staphylococcal alpha-toxin disrupts the barrier integrity of human intestinal epithelial Caco-2 cells as evidenced by decreased transepithelial electrical resistance (TER) and reduced cellular levels of junctional proteins, such as ZO-1, ZO-3, and E-cadherin. The Caco-2 cells also responded to alpha-toxin with an elevated cytosolic calcium ion concentration ([Ca(2+)](i)), elicited primarily by calcium influx from the extracellular environment, as well as with a significant reduction in TER, which was modulated by intracellular calcium chelation. Moreover, a significantly larger reduction in TER and amounts of the junctional proteins, viz., ZO-3 and occludin, was achieved by basolateral than by apical application of the alpha-toxin. These experimental findings thus support the hypothesis that free staphylococcal alpha-toxin in the bloodstream may cause intestinal epithelial barrier dysfunction and further aggravate the septic condition by promoting the release of intestinal bacteria into the underlying tissues and the blood.