How we fight death

Which hypotheses do we test first, and who decides?

How do we choose which hypotheses to test first? And how is that decided?

The engine does not pick a favourite. For every place where knowledge runs out it writes the whole field of rival explanations, binds each pair to the one observation that would separate them, and ranks the experiments rather than the ideas. The stem-cell niche below is the worked example: five rivals, one contested signal, one experiment that would knock some of them out.

A hypothesis tested first buys years. A hypothesis defended first costs them.

What we know

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.

clash: two established results disagree

Explanations

Rivals for the same gap, each stating what would knock it out

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

All questions