Structure and Function in Antimicrobial Piscidins: Histidine Position, Directionality of Membrane Insertion, and pH-Dependent Permeabilization

Piscidins are histidine-enriched antimicrobial peptides that interact with lipid bilayers as amphipathic α-helices. Their activity at acidic and basic pH in vivo makes them promising templates for biomedical applications. This study focuses on p1 and p3, both 22-residue-long piscidins with 68% sequence identity. They share three histidines (H3, H4, and H11), but p1, which is significantly more permeabilizing, has a fourth histidine (H17). This study investigates how variations in amphipathic character associated with histidines affect the permeabilization properties of p1 and p3. First, we show that the permeabilization ability of p3, but not p1, is strongly inhibited at pH 6.0 when the conserved histidines are partially charged and H17 is predominantly neutral. Second, our neutron diffraction measurements performed at low water content and neutral pH indicate that the average conformation of p1 is highly tilted, with its C-terminus extending into the opposite leaflet. In contrast, p3 is surface bound with its N-terminal end tilted toward the bilayer interior. The deeper membrane insertion of p1 correlates with its behavior at full hydration: an enhanced ability to tilt, bury its histidines and C-terminus, induce membrane thinning and defects, and alter membrane conductance and viscoelastic properties. Furthermore, its pH-resiliency relates to the neutral state favored by H17. Overall, these results provide mechanistic insights into how differences in the histidine content and amphipathicity of peptides can elicit different directionality of membrane insertion and pH-dependent permeabilization. This work features complementary methods, including dye leakage assays, NMR-monitored titrations, X-ray and neutron diffraction, oriented CD, molecular dynamics, electrochemical impedance spectroscopy, surface plasmon resonance, and quartz crystal microbalance with dissipation.

Bacteria-associated hemophagocytic syndrome

Histiocytic medullary reticulosis (HMR) was originally defined as a neoplastic disorder. Some cases reported as HMR have been characterized by a systemic proliferation of mature histiocytes showing hemophagocytosis, bone marrow necrosis, pancytopenia, hepatitis, and coagulopathy. Clinically, these patients have fever and constitutional symptoms and often have hepatosplenomegaly and lymphadenopathy. Although there is a high mortality rate, this process appears to be reactive and has been associated with active viral infection. Similar cases have been briefly described that were associated with other agents or disease processes, but concomitant viral infections were not excluded. Three characteristic examples of this hemophagocytic syndrome that were associated with bacterial sepsis are described. Active infection by those viruses that have previously been associated with the syndrome was excluded. It appears that the hemophagocytic syndrome may be associated with various types of active disseminated infections.