Single-Virus Fusion Measurements Reveal Multiple Mechanistically Equivalent Pathways for SARS-CoV-2 Entry.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binds to cell surface receptors and is activated for membrane fusion and cell entry via proteolytic cleavage. Phenomenological data have shown that SARS-CoV-2 can be activated for entry at either the cell surface or in endosomes, but the relative roles in different cell types and mechanisms of entry have been debated. Here, we used single-virus fusion experiments and exogenously controlled proteases to probe activation directly. We found that plasma membrane and an appropriate protease are sufficient to support SARS-CoV-2 pseudovirus fusion. Furthermore, fusion kinetics of SARS-CoV-2 pseudoviruses are indistinguishable no matter which of a broad range of proteases is used to activate the virus. This suggests that the fusion mechanism is insensitive to protease identity or even whether activation occurs before or after receptor binding. These data support a model for opportunistic fusion by SARS-CoV-2 in which the subcellular location of entry likely depends on the differential activity of airway, cellsurface, and endosomal proteases, but all support infection. Inhibition of any single host protease may thus reduce infection in some cells but may be less clinically robust. IMPORTANCE SARS-CoV-2 can use multiple pathways to infect cells, as demonstrated recently when new viral variants switched dominant infection pathways. Here, we used single-virus fusion experiments together with biochemical reconstitution to show that these multiple pathways coexist simultaneously and specifically that the virus can be activated by different proteases in different cellular compartments with mechanistically identical effects. The consequences of this are that the virus is evolutionarily plastic and that therapies targeting viral entry should address multiple pathways at once to achieve optimal clinical effects.

Early detection of cryostorage tank failure using a weight-based monitoring system.

PURPOSE: To study the relationship between liquid nitrogen loss and temperature in cryostorage dewars and develop an early-warning alarm for impending tank failure. METHODS: Cryostorage dewars were placed on custom-engineered scales, and weight and temperature data were continuously monitored in the setting of slow, medium, and fast rate-loss of LN2 to simulate three scenarios of tank failure. RESULTS: LN2 Tank weights and temperatures were continuously monitored and recorded, with a calculated alarm trigger set at 10% weight loss and temperature of - 185 °C. With an intact tank, a 10% loss in LN2 occurred in 4.2-4.9 days. Warming to - 185 °C occurred in 37.8-43.7 days, over 30 days after the weight-based alarm was triggered. Full evaporation of LN2 required ~ 36.8 days. For the medium rate-loss simulation, a 10% loss in LN2 occurred in 0.8 h. Warming to - 185 °C occurred in 3.7-4.8 h, approximately 3 h after the weight-based alarm was triggered. For the fast rate-loss simulation, a 10% weight loss occurred within 15 s, and tanks were depleted in under 3 min. Tank temperatures began to rise immediately and at a relatively constant rate of 43.9 °C/h and 51.6 °C/h. Temperature alarms would have sounded within 0.37 and 0.06 h after the breech. CONCLUSIONS: This study demonstrates that a weight-based alarm system can detect tank failures prior to a temperature-based system. Weight-based monitoring could serve as a redundant safety mechanism for added protection of cryopreserved reproductive tissues.