Metabolic-Homeostatic Axis

Systemic insulin resistance, mitochondrial decline, and brain bioenergetic failure converge on the same PI3K/AKT/mTOR and TGF-β/SMAD signaling pathways that maintain microglial homeostatic identity and neuronal metabolic fitness. AD bioenergetic hypometabolism and psychiatric metabolic inflexibility are proposed as age-dependent trajectories from a shared collapse of dynamic metabolic flexibility.

Core Claim

Attack/failure dichotomies at every level of the AD microglial and psychiatric literature resolve into a single upstream event: collapse of the metabolically-maintained homeostatic state. The TGF-β-dependent microglial signature (Butovsky), TREM2-dependent lipid-sensing and phagocytic fitness (Colonna), neuronal oxidative phosphorylation (Swerdlow mitochondrial cascade), and peripheral insulin sensitivity all run through overlapping PI3K/AKT/mTOR signaling and shared mitochondrial substrates. Loss at any point propagates through the network.

Converging Mechanisms

• TGF-β/SMAD failure drives loss of the Butovsky homeostatic microglial signature (von Bernhardi; see Homeostatic Microglial Collapse)
• TREM2 → DAP12 → SYK → PI3K/AKT/mTOR gates microglial metabolic fitness and phagocytic salvage (Colonna)
• Mitochondrial cascade sets age-dependent threshold for neuronal bioenergetic collapse (Swerdlow)
• Selfish brain + allostatic load bidirectional loop: brain ATP deficit → glutamatergic hyperexcitability → HPA/sympathetic activation → peripheral IR → chronic hyperglycemia that fails to rescue the brain (Peters; Sarnyai; Sethi et al. 2026)
• Dynamic metabolic inflexibility as lifespan phenotype: hyperglycolytic BD mania, hypometabolic AD, and SZ energy deficits are trajectories from a shared loss of metabolic switching capacity (Sarnyai & Ben-Shachar 2024)
• PV+ interneuron vulnerability as the convergence substrate: the highest-firing, most energy-demanding cells in cortex, ensheathed by PNNs, are the first to fail under combined metabolic and microglial stress (Kann 2014; Crapser 2020; de Vries 2024)

Key Concept

Metabolic collapse and microglial homeostatic collapse are not parallel processes — they are signaling-convergent. Systemic insulin resistance, through the same PI3K/AKT/mTOR pathway TREM2 uses, plausibly drives microglia out of the Butovsky homeostatic state; ketogenic therapy, GLP-1 agonism, metformin, and pioglitazone may all act upstream of the attack/failure dichotomy by stabilizing the metabolic substrate that maintains homeostasis in the first place.

Related Nodes

• Bioenergetics — the concept-level substrate this node elevates to a convergence axis
• Neuroimmune Interface — microglial homeostatic collapse is the myeloid projection of this axis
• Endosomal Nexus — v-ATPase and lysosomal acidification depend on ATP supplied by this axis
• Compensatory Paradigm Nexus — metabolic switching to aerobic glycolysis is an initially adaptive compensation that fails

Anchor Researchers (Prize Corpus)

• Russell Swerdlow — Mitochondrial Cascade Hypothesis
• Li-Huei Tsai — 40Hz gamma entrainment → mitochondrial ATP rescue
• Zaven Khachaturian — ATP-dependent calcium pump failure
• (to add) Butovsky, Colonna, von Bernhardi from the Homeostatic Collapse thesis

External References

• Sethi, S. et al. (2026). Metabolic psychiatry targeting metabolic dysregulation in mental health. Nature Mental Health. doi:10.1038/s44220-026-00609-5
• Sarnyai, Z. & Ben-Shachar, D. (2024). Schizophrenia, a disease of impaired dynamic metabolic flexibility. Psychiatry Res 342, 116220.
• Campbell, I. H. & Campbell, H. (2024). The metabolic overdrive hypothesis. Mol Psychiatry.
• Nørgaard, C. H. et al. (2022). GLP-1 receptor agonists and incidence of dementia (pooled RCTs, n=15,820). Alzheimers Dement 8, e12268.
• Cummings, J. L. et al. (2026). Efficacy and safety of oral semaglutide 14 mg in early-stage symptomatic Alzheimer's disease (evoke and evoke+): two phase 3, randomised, placebo-controlled trials. Lancet 407, 2167–2179. doi:10.1016/S0140-6736(26)00459-9 — negative; supersedes the Nørgaard reading for symptomatic disease.
• Ene, H. M. et al. (2023). Mitochondria play an essential role in schizophrenia-like neurodevelopment (mitochondrial transplantation). Mol Psychiatry 28, 1170.

Open Questions / Gaps

1. Is microglial homeostatic collapse downstream of systemic insulin resistance? Does peripheral IR propagate through brain IR → PI3K/AKT failure → TREM2 metabolic fitness loss → DAM salvage failure? Not currently wired in the KB.
2. GLP-1 agonists as homeostatic restoration — now a narrower question. Nørgaard 2022 reported lower dementia incidence with semaglutide/liraglutide in pooled trial and registry data, but the prospective test in established disease has failed: evoke and evoke+ randomised 3,808 amyloid-confirmed early-AD participants to oral semaglutide 14 mg or placebo for up to 156 weeks and found no separation on CDR-SB at week 104 (evoke: −0.08, 95% CI −0.35 to 0.20, P=0.57; evoke+: 0.10, 95% CI −0.17 to 0.38, P=0.46), both trials stopping for negative clinical outcome (Cummings 2026). The live question is whether GLP-1 agonism does anything for microglial homeostasis or PNN preservation before amyloid positivity — neither readout was measured in evoke, and a null on CDR-SB in symptomatic disease does not test it.
3. Ketogenic therapy × PNN integrity. Endorsed in both Swerdlow and Sethi literatures but never measured against the PV+/PNN biomarker the Homeostatic Collapse thesis proposes.
4. BAMs as systemic-metabolism sensors. Border-associated macrophages sit at the vascular interface and are the most likely first responders to systemic IR and dyslipidemia — unexamined in Prinz/Kierdorf.
5. Psychiatric → AD trajectory. If metabolic inflexibility is a lifespan phenotype, SZ/BD/MDD cohorts should show elevated AD conversion — testable in existing registries.
6. Mitochondrial transplantation in microglia. Ene 2023 showed feasibility in SZ models. Not proposed for restoring TREM2-dependent microglial fitness in AD.
7. PDH complex as AD node? Campbell & Campbell 2019 proposed pyruvate dehydrogenase deficiency for BD. PDH sits at the glycolysis/TCA junction repeatedly implicated in AD (acetyl-CoA → TCA; acetylcholine synthesis) but is not tagged in the KB.
8. Why do anti-inflammatory and metabolic trials fail the same way? Both may be downstream of the upstream PI3K/AKT/mTOR collapse this node identifies — predicts combined upstream targeting will outperform either alone.