Q&A with Junying Yuan.
Q: These days, active compounds tend to get checked for their commercial potential before they are openly published with their chemical structure, as salubrinal is. Could salubrinal be a candidate therapeutic molecule? If no, why not?
A: We don't propose that salubrinal is a drug candidate (or even a lead molecule), and we haven't looked into any of the parameters that good drugs need to have, such as its pharmacokinetic or pharmacodynamic properties in animals. However, we think that since salubrinal can affect ER stress-related and virus-related dephosphorylation pathways in a novel way, it provides an exciting proof-of-principle that future drugs might be able to take advantage of these pathways, as well. Salubrinal itself will probably be most useful as a research tool.
Q: An EC50 of 15 micromolar is generally considered not good enough for drug development. Are you working with follow-up compounds that bring this number into the nanomolar range?
A: Our collaborators at the Shanghai Institute of Organic Chemistry have synthesized a number of derivates of salubrinal that we've tested for things like stronger protection from apoptosis or lower EC50 values. So far, we haven't seen a dramatic improvement along those lines (with about 60 compounds tested). It's worth noting, though, that rescuing a cell from intense ER stress caused by strong poisons like tunicamycin is very difficult, so salubrinal is
unique in this regard, even though its EC50 isn't as low as we might like.
Q: Many neurodegenerative diseases are thought to be protein
conformation/misfolding diseases. Would cytoprotective compounds that alleviate the downstream effects of protein misfolding be general treatments for several diseases at once, or would they have to be specific to the given misfolding protein, i.e., Ht, ataxin, Aβ, α-synuclein, tau?
A: That was certainly part of the original rationale for starting our project, and many other labs have published evidence suggesting that correcting protein misfolding might be a productive therapeutic strategy in diverse neurodegenerative disorders.
Hopefully, compounds like salubrinal that manipulate protein folding and processing pathways will contribute to the development of future drugs targeting these processes.
Q: There is ongoing research on ER stress and the UPR in Alzheimer research, but no clear picture yet on where it fits into AD pathogenesis, especially in vivo. Where would salubrinal fit in?
A: We haven't looked into whether salubrinal might be useful in Alzheimer's model systems yet, but it would certainly be interesting and easy to test. There are some unpublished data for a potentially neuroprotective role of salubrinal from our collaborator’s lab, but we are not in a position to reveal them.
Q: Are any of the pathways you experimented with—eIF2α, PP1/GADD34,CReP—active in stressed neurons? Are they expressed in adult brain?
A: All these proteins are expressed in the adult brain, and many studies in the literature have indicated that ER stress pathways (including eIF2α phosphorylation) occurs in response to brain injury, such as acute ischemia. What's not yet clear is whether the induction of these pathways is a protective or destructive response to the injury, but we hope reagents like salubrinal might help answer this question.
Q: What are the next experiments to define the link to age-related neurodegeneration?
A: It would be certainly interesting to look into the possibility if Aβ-induced neuronal cell death can be protected by salubrinal.
Q: You describe initial results whereby topical salubrinal treatment reduced virus titer in eye swabs of eight mice whose corneas were infected with HSV. Your paper mentions potential uses of drugs like salubrinal in neurodegeneration in general terms. Can you expand on that?
A: As you mentioned before, protein misfolding and/or aggregation occurs in many neurodegenerative disorders and can disrupt multiple cellular organelles and processes, including the ER and secretory system. Knowing that, we hope that compounds like salubrinal could be used to manipulate these pathways and perhaps protect cells from the toxic effects of protein misfolding. Even though we don't consider
salubrinal a drug candidate, we think its novel mode of action might point in a useful direction for new drugs to be developed.
Q: Have you tried salubrinal in any assays with a neurodegeneration readout?
A: Not yet.
Q: Does it cross the blood-brain barrier? Could it be infused into AD/PD/HT models? Applied to slice cultures?
A: We haven't checked any of these things yet.
Q: Just how selective and safe is salubrinal? Your paper shows it does inhibit several PP1 substrates other than eIF2α, after all.
A: We think it's likely that salubrinal affects other
dephosphorylation events in the cell besides eIF2α, but probably not many. It seems clear that it doesn't affect all or even most PP1-mediated dephosphorylations. (Evidence for that is presented in the paper.) Also, the compound seems non-toxic to most cells at concentrations significantly higher than what's needed for cytoprotection, which at least suggests that inhibiting eIF2α dephosphorylation to a useful extent isn't necessarily toxic. One mystery we'd love to solve is to figure out exactly how, mechanistically, salubrinal is able to inhibit only a small subset of eIF2α dephosphorylations, but we don't know that yet.