Hypothesis

Von Economo neurons (VENs) in the frontoinsular cortex experience age‑related mitochondrial inefficiency that precedes detectable interoceptive decline, and this energetic deficit directly reduces autonomic output measured by heart‑rate variability (HRV) and interoceptive sensibility scores (MAIA‑6M).

Mechanistic Model

VENs possess a large, bipolar morphology enabling rapid glutamatergic signaling to autonomic nuclei [3]. This morphology entails high axonal conduction velocity and sustained firing rates, creating a substantial ATP demand. Recent single‑cell data show a global downregulation of ribosomal and metabolic genes (e.g., RPS27A, RPL3) in aged prefrontal neurons [5], suggesting a translational capacity shortfall that would disproportionately affect high‑output cells like VENs. We propose that, with age, VENs accumulate somatic mutations in transcription‑linked Signature A1 at ~13.8 SNVs/year [5], further compromising mitochondrial biogenesis genes (e.g., TFAM, NRF1). The resulting mitochondrial ROS elevation impairs calcium buffering, leading to episodic depolarization block and reduced glutamatergic drive to the dorsal vagal complex and sympathetic outflow centers. Consequently, autonomic flexibility—indexed by HRV—declines, weakening the afferent signals that underlie interoceptive awareness and body trust measured by MAIA‑6M [4].

Predictions

1. In cognitively normal adults aged 50‑80, VEN density (estimated via high‑resolution MRI‑based morphometry of the frontoinsular cortex) will correlate negatively with mitochondrial DNA mutation load in post‑mortem insular tissue (available from brain banks).
2. Lower VEN density will predict reduced HRV (RMSSD) and lower MAIA‑6M scores, independent of gray‑matter volume atrophy.
3. Longitudinally, individuals who exhibit a >10% drop in HRV over two years will show a subsequent 0.3‑unit decline in MAIA‑6M body‑trust subscale, mediated by baseline VEN density.
4. Exposing human cerebral organoids engineered to enrich for VEN‑like neurons (using FEZF2 and SOX5 over‑expression) to NAD+ precursors (e.g., nicotinamide riboside) will rescue mitochondrial membrane potential and restore spontaneous calcium transients compared with untreated aged organoids.

Experimental Design

• Human cohort: Recruit 120 participants, acquire 7‑T MRI for cortical thickness, collect ECG for HRV, administer MAIA‑6M, and collect peripheral blood for cfDNA mutation burden as a proxy for brain somatic load. Use structural equation modeling to test the mediation path VEN density → mitochondrial load → HRV → MAIA‑6M.
• Post‑mortem validation: In a subset of donors (n=30), perform stereological VEN counts in layer 5 of the frontoinsular cortex, quantify mtDNA copy number and SNV load via laser‑capture microdissection and duplex sequencing.
• Organoid assay: Generate VEN‑enriched organoids from iPSCs of young and old donors, treat with NAD+ booster or vehicle for 14 days, measure Seahorse OCR, MitoSOX ROS, and GCaMP calcium events. Compare across age and treatment groups using two‑way ANOVA.

Falsifiability

If VEN density shows no correlation with mitochondrial mutation load or HRV, or if NAD+ supplementation fails to improve mitochondrial metrics in VEN‑enriched organoids, the hypothesis would be refuted. Conversely, confirming the predicted mediation chain would substantiate VEN metabolic stress as a mechanistic bridge between cellular aging and systemic interoceptive decline.