How we fight death
Which hypotheses do we test first, and who decides?
The decision this question informs
Choose the uncertainty to resolve next
Choose tests for how much they can change a decision. Compare the predictions of several explanations before committing to one.
What should happen next?
Unresolved: Which feasible observation would separate the explanations, and would that distinction actually change the next intervention or study?
Next work: Compare the published predictions in the linked research example. Define shared conditions, controls and a decision rule before treating them as a study design.
What would change the direction: If explanations predict the same observation, the proposed test needs revision. If a result would not change the programme, another uncertainty may deserve priority.
Proposed agenda, not a statement that a study is underway.
Then the priority changes: A comparison may reduce uncertainty between the explanations.
What this does not establish: Different predictions alone do not make the test valid or decisive.
The research example: starting claims
Two S-nodes predict opposite TGF-β1 outcomes from the same SPM intervention in human macrophages.
What remains uncertain
No human niche macrophage data on IRF4-TGF-β1 co-binding status during SPM-driven polarization; intervention direction is indeterminate.
What is needed: Macrophage polarization state that clears senescent debris without TGF-β1-driven fibroblast activation.
two established results disagree
The science under it
The open questions this decides between
Explanations
Compare the explanations and their predictions
These candidates address a shared gap. Some mechanisms may coexist; this selection need not exhaust the possibilities. Their predictions describe proposed tests, not observed results.
- 01Resolvins may drive fibrosis in aged cells rather than resolve it
A worn-out FPR2 receptor does not merely respond weakly to resolvins. It switches them to a β-arrestin-biased output that reinforces TGF-β1 production, which would make supplementing aged tissue with these pro-resolution molecules actively harmful rather than simply ineffective.
In aged human macrophages with documented low FPR2 surface density, resolvin D1 treatment will increase TGF-β1 secretion above untreated controls (not just fail to decrease it), and this increase will be blocked by β-arrestin-2 siRNA but not by pertussis toxin (Gαi inhibitor), confirming β-arrestin-biased signaling as the driver.
Untested - 02NAD+ decides whether a healing macrophage also turns fibrotic
While the metabolic cofactor NAD+ is plentiful, the enzyme SIRT6 holds the TGFB1 promoter closed and a macrophage can resolve inflammation without driving scar. Aged macrophages lose most of their NAD+, the brake comes off, and restoring it should put the brake back.
Pre-treatment of aged human macrophages with NMN (1mM, 48h) before SPM-driven M2 polarization will reduce TGF-β1 secretion by >50% compared to vehicle, and this effect will be abolished by SIRT6 inhibitor OSS_128167. ChIP-qPCR will show restored H3K9 deacetylation at the TGFB1 promoter in NMN-treated aged macrophages.
Untested - 03The link between resolving inflammation and fibrosis is an artefact of culture plastic
Culture plastic is a million times stiffer than tissue, and that stiffness alone pushes the mechanical sensor YAP into the nucleus, where it switches on TGFB1 beside the healing programme. On tissue-soft gels the coupling should vanish, which would mean it never existed in a living niche in either species.
Human monocyte-derived macrophages polarized with resolvin D1 on 2 kPa polyacrylamide gels will show <2-fold TGF-β1 increase vs. unpolarized controls, while the same cells on tissue culture plastic show >5-fold increase. YAP nuclear/cytoplasmic ratio will correlate with TGF-β1 output (r>0.7) across substrate stiffnesses.
Untested - 04The fibrosis signal is stopped by the hyaluronan mesh, not by the macrophage
TGF-β1, the growth factor that turns fibroblasts into scar-forming cells, never reaches them while the dense ultra-high-molecular-weight hyaluronan coat around the cell is intact. What changes with age is the size of that mesh, not how much the macrophage secretes.
In human niche explant co-cultures, adding exogenous ultra-HMW-HA (>6 MDa) to the macrophage-fibroblast interface will block TGF-β1-driven αSMA expression in fibroblasts even when macrophages are maximally M2-polarized with high TGF-β1 secretion, while low-MW HA (<500 kDa) will permit fibroblast activation at the same TGF-β1 concentration.
Untested - 05Ageing installs the anti-fibrotic switch that spiny mice are born with
Constant itaconate in aged niche macrophages blocks TET2 and methylates the IRF4 site at the TGFB1 enhancer, so pro-resolution signalling stops driving scar formation. The gap between humans and the regenerating spiny mouse Acomys would then be a difference of age, not of species.
Bisulfite sequencing of the IRF4 half-site CpG at the TGFB1 enhancer in FACS-sorted CD163+ niche macrophages will show >60% methylation in aged (65-75) human donors but <10% in young (20-30) donors, and this methylation will correlate inversely with TGF-β1 secretion upon SPM stimulation in ex vivo polarization assays.
Untested
The next missing piece
From predictions to an experiment
No published experiment is linked to this question yet. The predictions above are material for designing a comparison. Shared conditions, controls, feasibility and an interpretation of null results still need to be specified.
Continue the reasoning