Compartment‑Specific NAD+ Augmentation Coupled to NDUFAF2 Phosphorylation Drives Respirasome Assembly and Mitigates Reductive Stress in Aging Muscle

Hypothesis

Targeted elevation of mitochondrial NAD+ via a mitochondrially‑restricted Nmnat3 fusion, combined with pharmacological inhibition of the NADH‑sensitive kinase that phosphorylates NDUFAF2, will increase respirasome (Complex I‑III‑IV supercomplex) stability, lower mitochondrial ROS, and improve ATP production in aged skeletal muscle.

Mechanistic Rationale

Declining nuclear NAD+ during aging reduces SIRT1 activity, stabilizes HIF‑1α, and creates a pseudohypoxic state that impairs oxidative phosphorylation [1]. While NAD+ precursors raise total cellular NAD+, they often fail to enrich the mitochondrial pool where electron transport chain (ETC) function resides [3]. Mitochondrial Nmnat3 overexpression selectively boosts mitochondrial NAD+, alters ETC protein composition, and reduces ROS in aged mice [5], yet the downstream effect on supercomplex assembly remains unclear.

The NADH/NAD+ ratio directly influences Complex I activity: excess NADH causes reductive stress, elevating ROS when electrons back up at the flavin site [4]. NDUFAF2, an assembly factor for Complex I, is phosphorylated by a NADH‑activated kinase (e.g., PKCδ) that reduces its affinity for nascent Complex I subunits, impairing respirasome formation. We propose that high mitochondrial NAD+ lowers NADH, decreasing kinase activity, thereby promoting NDUFAF2 dephosphorylation and stabilizing its interaction with Complex I. This dual action—more NAD+ for redox balance and less inhibitory phosphorylation on NDUFAF2—should favor respirasome assembly, minimize electron leak, and enhance ATP synthesis.

Testable Predictions

1. In aged mouse skeletal muscle, mitochondrially targeted Nmnat3 (mt‑Nmnat3) will increase mitochondrial NAD+ by ≥30 % without altering cytosolic or nuclear NAD+ pools.
2. Inhibition of the NADH‑sensitive kinase (using a selective PKCδ inhibitor) will reduce NDUFAF2‑Ser/Thr phosphorylation by ≥40 % in mt‑Nmnat3‑expressing muscle.
3. Combined mt‑Nmnat3 expression + kinase inhibition will raise respirasome abundance (measured by BN‑PAGE) by ≥25 % compared with either manipulation alone.
4. Mitochondrial ROS (MitoSOX fluorescence) will drop ≥35 % and ATP production rates (Seahorse XF) will rise ≥20 % in the combined treatment group relative to aged controls.
5. Functional outcomes—grip strength and treadmill endurance—will improve significantly only when both mitochondrial NAD+ is elevated and NDUFAF2 phosphorylation is suppressed.

Experimental Approach

• Generate an AAV9 vector encoding mt‑Nmnat3 fused to a COX8 mitochondrial targeting sequence; inject into tibialis anterior of 24‑month‑old C57BL/6 mice.
• Treat cohorts with a PKCδ inhibitor (e.g., rottlerin analog) or vehicle via osmotic pump for 4 weeks.
• Quantify subcellular NAD+ pools using enzymatic cycling assays isolated mitochondria, nuclei, and cytosol.
• Assess NDUFAF2 phosphorylation via phospho‑specific antibodies and mass spectrometry.
• Evaluate respirasome assembly by blue‑native PAGE followed by immunoblot for Complex I, III, IV subunits.
• Measure mitochondrial ROS with MitoSOX and ATP output using Seahorse XF Analyzer.
• Perform grip‑stick and incremental treadmill tests to gauge physiological improvement.

Falsification: If mt‑Nmnat3 alone or kinase inhibition alone does not increase respirasome stability, or if the combined treatment fails to reduce ROS or improve ATP output despite verified NAD+ elevation and NDUFAF2 dephosphorylation, the hypothesis would be refuted.