The last prediction
Back in Part 3, the Wisconsin team made a clear prediction. If SS-31 acts like a broken DRP1, holding cardiolipin in the inner membrane, then SS-31 treated cells should show three things:
- Less BAX gathering on mitochondria
- Less cytochrome c escaping
- Less fragmentation of mitochondria at the end of the process
Part 6 dealt with the first two. BAX still gathered in full, and cytochrome c still escaped in every cell. That leaves fragmentation.
Measuring fragmentation
The team measured the average size of mitochondria in each cell at three stages:
- Phase 1: at least 20 minutes before BAX started gathering
- Phase 2: while BAX was gathering
- Phase 3: in the 20 minutes after BAX finished gathering
If SS-31 reduced fragmentation, mitochondria in treated cells should stay larger in Phase 3.
What they found
There was no difference between SS-31 and vehicle cells at any stage. Not before BAX gathered, not during, not after.
All groups showed the same pattern: a clear, significant drop in mitochondrial size after BAX gathering was complete. The mitochondria broke apart just as much with SS-31 as without it.
None of the three predictions held
So, in summary:
| Prediction if SS-31 acts like broken DRP1 | Result |
|---|---|
| Less BAX gathering | No, same total amount |
| Less cytochrome c release | No, released in every cell |
| Less fragmentation | No, same fragmentation |
The authors write it plainly: SS-31 failed to induce any of these effects in their experiments. Their conclusion is that SS-31 works by a mechanism independent of DRP1.
Why "independent of DRP1" is useful news
This might sound like a negative result, but it actually fits neatly with something else in the research.
The paper points to an earlier study from the Reddy group, the same group behind the diabetic liver study in our other series. That study found that SS-31 and a DRP1 blocker called Mdivi1 worked together in an Alzheimer's model, with a combined protective effect greater than either alone.
If SS-31 worked by blocking the same thing as Mdivi1, combining them would not add much. You cannot block a door twice. But if SS-31 works through a different route, combining two separate protective effects makes sense.
So the Wisconsin finding, "SS-31 does not act like broken DRP1", is consistent with the Reddy group's finding that SS-31 and a DRP1 blocker add up. Two different labs, two different methods, a coherent picture.
What does SS-31 do, then?
Here is what this study leaves us with.
What SS-31 did:
- Helped mitochondria recover to larger, longer shapes faster (Part 5)
- Slightly slowed BAX gathering on the mitochondria (Part 6)
What SS-31 did not do, once apoptosis had started:
- Block or delay BAX
- Reduce total BAX
- Stop cytochrome c escaping
- Reduce fragmentation
The most sensible reading is that SS-31's benefits, where they exist, probably come earlier. Keeping the folded inner membrane stable, supporting energy production and reducing oxygen radical damage might keep a stressed cell from ever reaching the point where it sends the death signal.
Once the decision is made and BAX starts moving, SS-31 does not appear to be able to reverse it, at least in this system.
A useful way to think about it
Picture a building with a fire alarm system.
You could protect the building in two ways. You could disable the alarm, so even when a fire breaks out the evacuation never starts. Or you could fireproof the building, so fires are less likely to start and spread in the first place.
The earlier reading of SS-31 was closer to the first idea: stepping in to stop the death alarm. This study suggests it is closer to the second: supporting the building so the alarm is less likely to be triggered. Once the alarm is ringing, SS-31 does not silence it.
That is a more modest claim. It is also a more believable one, and it fits better with how SS-31 is described in most of the research, as something that supports the structure and function of mitochondria.
The authors' own open question
The paper is honest about what it does not know. The mechanism behind the small slowdown in BAX is unknown. The authors' suggestion, that SS-31 might partly reduce cardiolipin moving to the outer membrane, is labelled clearly as a hypothesis that remains to be tested.
I find that refreshing. A lot of writing about peptides fills every gap with confident explanations. This paper leaves the gap visible.
Next, in the final part, the funding question, and the real limits of what a study in cells can tell us.
This series discusses a peer-reviewed laboratory study in cultured cells. The results have not been shown in people. Nothing here is medical advice, and SS-31 is discussed for research purposes only.
