Hypothesis

Transient elevation of NAD+ levels combined with a short‑acting lamin A/C modulator will restore nucleocytoplasmic transport efficiency in iPSC‑derived neurons without erasing the age‑related molecular signatures that direct conversion preserves, thereby improving functional maturation while retaining disease‑relevant phenotypes.

Rationale

Directly converted induced neurons retain donor‑age hallmarks such as nucleocytoplasmic defects, which make them superior for modeling late‑onset pathology [4]. However, these cells often show immature electrophysiology and poor survival, limiting scalability [3]. iPSC‑derived neurons, although scalable, lose many age‑related marks during full reprogramming, which can blunt disease phenotypes [4]. Recent work shows that NAD+ depletion exacerbates lamin A/C‑dependent nuclear envelope stiffness and impairs transport of RNA‑binding proteins implicated in ALS and FTLD [5]. Conversely, boosting NAD+ with precursors like nicotinamide riboside improves mitochondrial function and reduces nuclear stress [1], while transient inhibition of lamin A/C farnesylation (e.g., with a low‑dose FTase inhibitor) increases nuclear envelope pliability without causing permanent laminopathy [2].

We hypothesize that a brief, coordinated pulse of NAD+ elevation and lamin A/C modulation will:

1. Increase nuclear pore complex flexibility, rescuing nucleocytoplasmic transport of TDP‑43, FUS, and other ALS/FTLD proteins.
2. Preserve epigenetic age markers (e.g., DNA methylation clocks, histone heterochromatin loss) because the intervention is metabolically, not genomically, focused.
3. Enhance downstream maturation signals (Ca2+ handling, synaptic gene expression) by alleviating nuclear stress, leading to better electrophysiological properties.
4. Remain compatible with existing differentiation protocols, allowing scale‑up under GMP conditions.

Experimental Design

• Cell lines: Generate iPSCs from healthy donors and from ALS patients with SOD1, C9orf72, or FUS mutations; also produce directly converted induced neurons (iN) from the same fibroblasts as a fidelity benchmark.
• Treatment: Apply nicotinamide riboside (500 µM) for 24 h combined with a reversible FTase inhibitor (tipifarnib at 100 nM) for 6 h, timed to coincide with the midpoint of neuronal maturation (day 14 of a dual SMAD‑inhibition protocol). Include vehicle controls and single‑agent arms.
• Readouts:
  • Nucleocytoplasmic transport: fluorescence recovery after photobleaching (FRAP) of NLS‑GFP and NES‑GFP reporters; subcellular fractionation of TDP‑43/FUS followed by western blot.
  • Age signatures: DNA methylation epigenetic clock (Horvath), senescence‑associated heterochromatin foci (SAHF) immunostaining, and transcriptomic clocks.
  • Maturation: patch‑clamp electrophysiology (action potential threshold, sodium/potassium current densities), synaptic marker synapsin‑1/PSD‑95 immunostaining, calcium imaging with Fluo‑4.
  • Disease phenotypes: neuronal hyperexcitability, stress‑induced granule formation, and susceptibility to excitotoxic challenge (glutamate dose‑response).
  • Scalability: flow‑cytometry purity of TUJ1+/MAP2+ cells, yield per µg starting plasmid, and viability after cryopreservation.
• Statistical plan: n = 3 biological replicates per condition, ANOVA with Tukey post‑hoc; significance set at p < 0.05.

Predicted Outcomes

If the hypothesis is correct, combined NAD+ + lamin A/C modulation will:

• Increase nuclear import/export rates by ≥30 % versus controls, matching levels seen in youthful fibroblasts.
• Leave epigenetic age acceleration scores unchanged (ΔAge < 0.5 years) relative to untreated iPSC‑neurons, while preserving the age‑related transcriptomic shifts seen in iN cells.
• Boost mature neuronal properties: action potential firing frequency ↑25 %, sodium current density ↑20 %, synaptic puncta density ↑35 %.
• Retain or enhance disease‑specific readouts: SOD1‑mutant neurons show unchanged misfolded aggregate load; C9orf72‑repeat neurons maintain comparable dipeptide‑protein synthesis; FUS‑mutant cells display similar stress‑granule dynamics.
• Improve manufacturing metrics: >80 % TUJ1+/MAP2+ purity, >1‑fold increase in viable cell yield after thaw, and sustained expression of neuronal markers over 30 days in culture.

Failure to observe rescue of transport, or a significant reduction in age‑signature metrics, would falsify the hypothesis and suggest that NAD+‑lamin A/C coupling does not selectively uncouple maturation from aging preservation.

Potential Pitfalls & Alternatives

• Toxicity from prolonged NAD+ elevation could artificially lower age markers; hence the limited 24‑h window and inclusion of a washout period.
• FTase inhibition may affect Ras signaling; using a low dose and short exposure minimizes off‑target effects, and we will monitor ERK phosphorylation as a safety readout.
• If single agents suffice, the factorial design will reveal synergy; lack of additivity would prompt testing other nuclear envelope modulators (e.g., LAP2α peptides) or sirtuin activators.

This framework directly tests whether a metabolically targeted, transient intervention can bridge the gap between the scalability of iPSC‑derived neurons and the aging fidelity of directly converted cells, offering a concrete path toward clinically viable, disease‑specific neuronal products for neurodegenerative disease modeling and therapy.