Regulation of epitope exposure in the gp41 membrane-proximal external region through interactions at the apex of HIV-1 Env

Glycoprotein Env of human immunodeficiency virus type 1 (HIV-1) mediates viral entry through membrane fusion. Composed of gp120 and gp41 subunits arranged as a trimer-of-heterodimers, Env adopts a metastable, highly dynamic conformation on the virion surface. This structural plasticity limits the temporospatial exposure of many highly conserved, neutralizing epitopes, contributing to the difficulty in developing effective HIV-1 vaccines. Here, we employed antibody neutralization of HIV-1 infectivity to investigate how inter- and intra-gp120 interactions mediated by variable loops V1/V2 and V3 at the Env apex regulate accessibility of the gp41 membrane-proximal external region (MPER) at the Env base. Swapping the V3 loop from Env_SF162 into the Env_HXB2 background shifted MPER exposure from the prefusogenic state to a functional intermediate conformation that was distinct from the prehairpin-intermediate state sensitive to gp41-targeted fusion inhibitors. The V3-loop swap had a profound impact on global protein dynamics, biasing the equilibrium to a closed conformation resistant to most anti-gp120 antibodies, stabilizing the protein to both cold- and soluble CD4-induced Env inactivation, and increasing the CD4 requirements for viral entry. Further dissection of the Env_HXB2 V3 loop revealed that residue 306 uniquely modulated epitope exposure and trimer stability. The R306S substitution substantially decreased sensitivity to antibodies targeting the gp41 MPER and, surprisingly, the gp120 V3-loop crown (residues 312–315), but had only modest effects on exposure of intervening gp120 epitopes. Furthermore, the point mutation reduced soluble CD4-induced inactivation, but had no impact on cold inactivation. The residue appeared to exert its effects by electrostatically modifying the strength of intra-subunit interactions between the V1/V2 and V3 loops. The distinct patterns of neutralization and stability pointed to a novel prefusogenic Env conformation along the receptor activation pathway and suggested that apical Env-regulation of gp41 MPER exposure can be decoupled from much of the dynamics of gp120 subunits.

Surface glycoprotein Env is the main target for neutralizing antibodies elicited by HIV-1 vaccines. Env spontaneously fluctuates among different structures, limiting exposure of many attractive antibody-binding epitopes and, thereby, confounding vaccine development. To characterize these fluctuations, we examined how exposure of the MPER epitope found at the base of Env is regulated by interactions of the V3 loop located in the apex. Starting with an extremely flexible Env with a readily-exposed MPER, we identified two alterations that substantially restricted antibody access to the epitope. The first, a wholesale swap of V3 loops between HIV-1 strains, energetically stabilized Env in a closed structure that restricted access to antibodies throughout the protein. The second, a point mutation that altered V3-loop charge, specifically destabilized the MPER-exposed conformation but had minimal impact on antibody access to Env regions in between the apex and base. The results indicate that MPER exposure is not explicitly tied to the dynamics of Env regions between the apex and base and suggest a new structural fluctuation during Env activation.

Incidence of visceral pain during cesarean section: the effect of varying doses of spinal bupivacaine

The safety of 0.5% hyperbaric bupivacaine, as well as the incidence and severity of visceral pain, were evaluated in 36 women undergoing elective cesarean section under spinal anesthesia who, randomly divided into two groups, received different dose ranges according to height, 7.5-10 mg in group A and 10-12.5 mg in group B. When sensory block to at least the fourth thoracic dermatome was established, surgery was begun and the occurrence and severity of visceral pain recorded (visual analog scale) by an observer unaware of patient data. The level of analgesia to pinprick was determined when and if there was onset of pain intraoperatively, and supplementary medication was administered as needed. Hypotension, the incidence of which was similar in both groups, was treated as necessary with ephedrine. No patients experienced pain until after delivery of the infant. Thereafter, moderate to severe pain, in association with peritoneal traction, occurred in 12 patients in group A (70.5%) but only in 6 patients in group B (31.6%). In patients experiencing moderate to severe pain, the mean time between induction of anesthesia and onset of pain was similar in both groups, as was the amount of systemic narcotic given. Total time for regression of sensory analgesia to L5 was longer in patients in group B (243.9 versus 195.4 min), and the incidence of complete motor blockade was greater in group B. Increasing the amount of 0.5% hyperbaric bupivacaine per spinal segment reduces the occurrence of moderate to severe visceral pain during elective cesarean section without jeopardizing mother or fetus.

Adverse effects of maternally administered lidocaine on the asphyxiated preterm fetal lamb

Lidocaine was infused at a constant rate of 0.1 mg.kg-1.min-1 for 180 min into 12 chronically prepared pregnant sheep while asphyxia, induced by partial umbilical cord occlusion, was maintained in the premature fetus (80% of gestation). In five similar preparations saline instead of lidocaine was infused into the mother for 180 min. Maternal and fetal arterial blood pressure, heart rate, pHa, PaCO2, and PaO2 were monitored, and fetal cardiac output and the distribution of blood flow to fetal organs were measured, using labeled microspheres, before and after asphyxia and again after maternal infusion of lidocaine or saline. Maternal and fetal arterial blood and maternal urine were obtained at intervals for determination of lidocaine concentrations and urinary drug clearance. At the end of infusion, these animals were killed and tissues dissected for determination of lidocaine concentrations and organ blood flow. Maternal and fetal lidocaine plasma concentrations at steady state were 2.32 +/- 0.12 and 1.23 +/- 0.17 microgram/ml, respectively, similar to those seen during human epidural anesthesia. Asphyxia resulted in a significant drop in fetal heart rate and increased blood flow to the brain, heart, and adrenals. Asphyxia and saline did not produce additional deterioration of the fetus, but asphyxia and lidocaine led to a significant increase in PaCO2 and decreases in pHa, mean arterial pressure, and blood flows to the brain, heart, and adrenals. It is concluded that the immature fetus loses its cardiovascular adaptation to asphyxia when exposed to clinically acceptable plasma concentrations of lidocaine obtained transplacentally from the mother.