Hypothesis

Aging‑associated CD38 upregulation is not uniform; it concentrates in von Economo neurons (VENs) of the anterior insular cortex, creating a localized NAD+ sink that selectively compromises lysosomal autophagy and mitophagy in these cells. Because VENs rely on high NAD+‑dependent sirtuin activity to maintain their large, metabolically active morphology, CD38‑mediated NAD+ depletion triggers a feed‑forward loop: loss of SIRT3‑driven deacetylation impairs mitochondrial quality control, increasing ROS and DNA damage, which further activates PARP and amplifies NAD+ consumption. The resulting bioenergetic crisis reduces the neuron's capacity to sustain the high‑frequency firing that underlies interoceptive awareness, manifesting as the observed failure of acute NMN to improve interoceptive signals.

Mechanistic Insight

CD38 catalyzes the conversion of NAD+ to cyclic ADP‑ribose (cADPR) and ADP‑ribose, both of which can inhibit the lysosomal V‑ATPase proton pump when accumulated in the lumen (see [2] for CD38 enzymatic products). In VENs, the exceptionally high basal calcium flux through TRP channels makes them especially sensitive to cADPR‑mediated ER calcium release, which in turn drives lysosomal alkalinization and blocks autophagosome‑lysosome fusion. This mechanistic step is absent in most cortical neurons, explaining the selective vulnerability.

Testable Predictions

1. Spatial NAD+ mapping – Using NAD+‑specific fluorescent biosensors (e.g., SoNar) in aged mouse insular slices, NAD+ levels will be significantly lower in VEN‑rich layer 5b compared with neighboring pyramidal neurons ([1] shows tissue‑wide decline; we predict a cell‑type‑specific exaggeration).
2. CD38 colocalization – Immunohistochemistry for CD38 will show puncta tightly apposed to VEN membranes and lysosomal markers (LAMP1), whereas pyramidal cells display diffuse cytoplasmic signal ([5] demonstrated SASP‑driven CD38 induction; we predict a juxtaluminal pattern in VENs).
3. Pharmacological rescue – Selective CD38 inhibition (e.g., 78c) applied to aged insular slices will restore NAD+ specifically in VENs, normalize lysosomal pH (measured with LysoSensor), and rescue autophagic flux (LC3‑II turnover) without affecting global NAD+ levels ([7] shows systemic NAD+ restoration; we predict a cell‑type‑restricted effect).
4. Functional read‑out – In vivo fiber‑photometry of calcium transients in VENs during intra‑gastric distension will show blunted responses in aged mice; CD38 inhibition will recover the amplitude to youthful levels, correlating with improved performance in an interoceptive discrimination task ([6] reported NMN fails to enhance interoception; we predict CD38 blockade succeeds).

Experimental Approach

• Generate a VEN‑specific Cre line (e.g., FEZF2‑Cre) crossed with a floxed CD38 allele to delete CD38 selectively in VENs; compare NAD+ levels, lysosomal integrity, and interoceptive behavior to littermate controls.
• Use AAV‑delivered SoNar sensor under the FEZF2 promoter for longitudinal two‑photon imaging of NAD+ dynamics in awake mice undergoing aging.
• Apply lysosomal pH probes and autophagic reporters (mCherry‑GFP‑LC3) in acute slices to quantify flux before and after CD38 inhibitor treatment.
• Behavioral assay: train mice to discriminate mild vs. moderate gastric distension; measure choice accuracy and reaction time.

Falsifiability

If CD38 deletion or inhibition does not preferentially raise NAD+ in VENs, does not correct lysosomal pH, or fails to rescue interoceptive calcium signaling and behavior, the hypothesis is refuted. Conversely, a rescue limited to VENs would support the model.

Broader Implication

This mechanism reframes NAD+ decline not as a global metabolic downgrade but as a targeted sabotage of high‑hub interoceptive neurons, linking cellular senescence‑driven CD38 upregulation to the specific loss of subjective feeling that characterizes aging and neurodegenerative disease.