Reviewer #2 (Public Review):
The authors set out to study the potent HIV capsid inhibitor lenacapavir (LEN) and how it alters capsid stability. They use a previously developed single-molecule fluorescence imaging assay to take two measurements of individual viral particles over time: 1) they track the release of GFP from GFP-loaded particles to determine whether the capsid is intact or open, and 2) they track the disassembly of the capsid lattice by measuring the signal intensity of a capsid binding fluorophore (AF568-CypA), which diminishes as the capsid lattice subunits disassociate.
As in their previous work, the authors report that most of their capsids are "leaky" and rapidly lose GFP after the viral membrane is permeabilized, followed by disassembly of the capsid lattice. A subset of capsids maintain GFP signal for various periods of time until they spontaneously "open," and a smaller subset remains closed for the entire length of the imaging experiment (typically 30 min). Interestingly, the authors find that LEN has two effects in this assay: it not only promotes a more rapid release of GFP (interpreted to mean loss of capsid integrity), but it also prevents the capsid lattice from disassembling after opening. As expected, the cellular cofactor IP6 (which stabilizes capsids in cells and in vitro) was found to protect against capsid rupture and counteracted the effects of LEN (although high concentrations of LEN could override any protective effects of IP6).
Their single-molecule experiments are nicely buttressed by in vitro assembly reactions of purified CA protein, with IP6 promoting cone formation and LEN promoting aberrant assembly into tubes. The authors go further to test the kinetics of LEN's effects on HIV infection and reverse transcription, and they perform experiments in comparison to other factors that target the FG binding pocket (BI-2, PF-74, and a peptide from the host factor CPSF6). They find that LEN works differently than these other capsid binders, and stabilizes the lattice structure much more effectively, which the authors suggest is due to how well LEN bridges between CA-CA monomers and rigidifies CA hexamers.
It's particularly interesting that the results of their kinetic studies indicate that LEN's effects on capsid strain (which may ultimately promote rupture) may not happen immediately, but instead, take time to build as the drug occupies more and more binding sites. The authors estimate that roughly 30% of binding sites need to be occupied by LEN to reach half-maximal inhibition of infection, and based on their binding curves, it may take ~20h to reach this level of occupancy in the presence of sub nM concentrations of LEN. Although other mechanisms in addition to catastrophic rupture of capsids are likely at play during inhibition of infection (such as inhibition of host factor binding), these kinetics support previous reports that the most potent functions of capsid inhibition occur at or between the steps of nuclear entry and integration.
It is important to note that although in vitro uncoating assays can help us understand the physical nature of HIV capsid and capsid inhibitor interactions, the assays in this paper might not accurately model the capsid dynamics that are experienced in a cell during infection. The authors report that more than half of their capsids are "leaky" at the start of their assay, but this could be an artifact of the experimental system. Several groups have now demonstrated that capsids remain intact or largely intact for several hours after infection. Thus, while their method is valuable to the research community and can provide insight into capsid stability (and how it can be influenced by capsid binding factors), the authors should be cautious about using pore-forming proteins to permeabilize the virion and interpreting the release of GFP in their single-molecule fluorescence system as an accurate reflection of HIV dynamics in vivo.
In this regard, it would be helpful to establish whether the pore-forming proteins used in vitro to permeabilize the virus membrane have an impact on capsid integrity. It's possible that the concentration of pore-forming proteins used in this paper (200nM) actually promotes "leaky" capsids and rapid opening of capsids in vitro, whereas capsids in their native state in the cytoplasm could remain mostly intact until disrupted by host factors and/or small molecules. Determining whether lower concentrations of DLY/SLO (or PFO as used in Marquez et al., 2018) change the ratio of leaky to closed capsids, or delay the time to capsid opening (either in the presence of IP6 or in the presence of LEN) would be informative. It may be possible to optimize the concentration of pore-forming proteins (and other buffer constituents) to achieve permeabilization of the membrane with minimal disruption to capsid integrity, which could approximate conditions within the cell.
Experiments with capsid mutations that stabilize or destabilize the lattice structure (and exhibit different sensitivities to IP6) could help support the authors' conclusions, as would testing mutations that confer resistance to LEN (e.g. Q67H+N74D, M66I, etc...). It would be of great interest to find if CA mutations affect either GFP release or the CypA paint signal, and whether resistance mutations mitigate the effects of LEN in single-molecule experiments.
The discussion section of this paper is expertly written and places the work into the larger context of HIV research. The authors have thoughtfully analyzed their experiments with capsid inhibitors in relation to kinetics, occupancy, the potential for rigidification, and cofactor binding. They offer reasonable explanations for how LEN exhibits opposing effects on the HIV capsid at high occupancy through inducing capsid rupture while simultaneously preventing the dissociation of CA subunits. Many lines of evidence are now converging on the concept that the capsid evolved to be stable enough to protect its contents, yet flexible enough to navigate the steps of reverse transcription, nuclear entry, and uncoating. With this paper, the authors make a strong case that LEN functions as an antiviral, at least in part, through engaging "lethal hyperstabilization" of the capsid, promoting rigid lattice formations that are incompatible with closed cone structures.