Spiegel, F. | bioRxiv Science

bioRxiv2024

Strong family- and guild-specific responses of arbuscular mycorrhizal fungi to long-term deficiencies and imbalances of N, P and K

O_LIMany agroecosystems face nitrogen (N), phosphorus (P) or potassium (K) deficiencies due to imbalanced or insufficient nutrient replenishment after plant biomass harvest. How this affects the symbiosis between plants and arbuscular mycorrhizal fungi (AMF), and the abundance of exploration-based AMF guilds (i.e., rhizophilic, edaphophilic, ancestral) remains largely unknown. C_LIO_LIWe studied a 70-year nutrient-deficiency experiment in a managed grassland in central Austria, where aboveground biomass was harvested three times annually. N, P and K were fully, partially, or not replenished, causing long-term nutrient deficiencies and imbalances. We analysed AMF communities in soil and roots by DNA/RNA amplicon sequencing and fatty-acid biomarkers, alongside soil and plant community properties. C_LIO_LISoil AMF communities were affected by N and P deficiencies, while root AMF communities were most susceptible to K deficiency, showing a 50% biomass reduction. We observed distinct guild- and family-specific responses: The edaphophilic guild declined with N deficiency, while the rhizophilic guild decreased with P and K deficiencies. Families within each guild, particularly in the ancestral guild, showed differential responses, indicating complementary nutrient specializations at the family level. C_LIO_LIOur findings underscore the previously unrecognized role of K deficiency in AMF symbiosis and suggest the existence of nutrient-related functional subgroups within exploration-based AMF guilds. C_LI

ecology↗

bioRxiv2023

Role of lipid nanodomains for inhibitory FcγRIIb function

The inhibitory Fc{gamma} receptor Fc{gamma}RIIb is involved in immune regulation and is known to localize to specific regions of the plasma membrane called lipid rafts. Previous studies suggested a link between the altered lateral receptor localization within the plasma membrane and the functional impairment of the Fc{gamma}RIIb-I232T variant that is associated with systemic lupus erythematosus. Here, we conducted microsecond all-atom molecular dynamics simulations and IgG binding assays to investigate the lipid nano-environment of Fc{gamma}RIIb monomers and of the Fc{gamma}RIIb-I232T mutant within a plasma membrane model, the orientation of the Fc{gamma}RIIb ectodomain, and its accessibility to IgG ligands. In contrast to previously proposed models, our simulations indicated that Fc{gamma}RIIb does not favor a cholesterol-or a sphingolipid-enriched lipid environment. Interestingly, cholesterol was depleted for all studied Fc{gamma}RIIb variants within a 2-3 nm environment of the receptor, counteracting the usage of raft terminology for models on receptor functionality. Instead, the receptor interacts with lipids that have poly-unsaturated fatty acyl chains and with (poly-) anionic lipids within the cytosolic membrane leaflet. We also found that Fc{gamma}RIIb monomers adopt a conformation that is not suitable for binding to its IgG ligand, consistent with a lack of detectable binding of monomeric IgG in experiments on primary immune cells. However, our results propose that multivalent IgG complexes might stabilize Fc{gamma}RIIb in a binding-competent conformation. We suggest differences in receptor complex formation within the membrane as a plausible cause of the altered membrane localization or clustering and the altered suppressive function of the Fc{gamma}RIIb-I232T variant. Significance StatementOur study sheds new light on the molecular mechanisms underlying the regulation of immune signaling mediated by the inhibitory Fc{gamma} receptor (Fc{gamma}RIIb). By utilizing atomistic simulations and experimental assays, we demonstrate that Fc{gamma}RIIb interacts with specific lipids in the plasma membrane. Notably, our findings challenge the current view of membrane heterogeneity in immune cells, as Fc{gamma}RIIb is not localized in specialized membrane domains known as rafts. Rather, we propose that receptor complex formation modulates receptor localization and conformation, thereby enabling ligand binding. Our findings have important implications for understanding how immune receptors function and communicate with each other, and may provide new opportunities for developing therapeutic strategies targeting Fc{gamma}RIIb in diseases such as autoimmunity and cancer.

immunology↗