pH-dependent conformational change within the Lassa virus transmembrane domain elicits efficient membrane fusion

Lassa virus (LASV) is the most prevalent member of the arenavirus family and the causative agent of Lassa fever, a viral hemorrhagic fever. Although there are annual outbreaks in West Africa, and recently isolated cases worldwide, there are no current therapeutics or vaccines, which poses LASV as a significant global public health threat. One of the key steps in LASV infection is the delivery of its genetic material by fusing its viral membrane with the host cell membrane. This process is facilitated by significant conformational changes within glycoprotein 2 (GP2), yielding distinct prefusion and postfusion structural states. However, structural information is missing to understand the changes that occur in the transmembrane domain during the fusion process. Here, we used CD and NMR spectroscopy to show that the transmembrane domain has pH-dependent conformational changes that result in an extension of the alpha helix at the N-terminal end. Proline mutants of key residues in that region prevent the helical extension, as seen in CD and NMR. We developed a modified lipid mixing assay to study the importance of this extension on the function of GP2. Our assay shows that membrane fusion efficiency is optimal at low pH values but introducing the proline mutants results in lower fusion efficiency. These results indicate that these pH-dependent conformational changes are important to the fusion mechanism. This information can be used to design therapeutics to combat Lassa virus infections and prevent its potential spread.

Ciclopirox prevents peroxynitrite toxicity in astrocytes by maintaining their mitochondrial function: a novel mechanism for cytoprotection by ciclopirox

Previously we have reported that astrocytes deprived of glucose were highly vulnerable to peroxynitrite (Choi and Kim, J. Neurosci. Res. 54 (1998) 870; Neurosci. Lett. 256 (1988) 109; Ju et al., J. Neurochem. 74 (2000) 1989). Here we report that ciclopirox, which is clinically used as an anti-fungal agent, completely prevents the increased death in glucose-deprived astrocytes exposed to 3-morpholinosydnonimine (SIN-1, a peroxynitrite-releasing reagent). The increased vulnerability was in good correlation with the peroxynitrite-evoked decrease of mitochondrial transmembrane potential (MTP) in astrocytes. A simultaneous exposure to glucose deprivation and SIN-1 rapidly depolarized MTP and depleted ATP in astrocytes. Inclusion of ciclopirox initially increased the MTP, maintained it high, and blocked the ATP depletion in glucose-deprived SIN-1-treated astrocytes. However, ciclopirox did not prevent the depletion of reduced glutathione in glucose-deprived SIN-1-treated astrocytes. Consistently, ciclopirox did not scavenge various kinds of oxidants including peroxynitrite, nitric oxide, superoxide anion, hydrogen peroxide and hydroxyl radical. Ciclopirox has been experimentally used as a cell cycle G1/S phase transition blocker (Hoffman et al., Cytometry 12 (1991) 26). Flow cytometry analysis, however, showed that the cytoprotective effect of ciclopirox was not attributed to its inhibition of the cell cycle progression. The present results indicate that ciclopirox protects astrocytes from peroxynitrite cytotoxicity by attenuating peroxynitrite-induced mitochondrial dysfunction.