IF a triple-AAV vector system — (1) AAV2/7-GFAP-CasRx-gRNA-Ptbp1 at 2×10¹² vg/mL, (2) AAV2/7-GFAP-ASCL1SA6 (phospho-site-deficient, serine-to-alanine mutations at all six CDK phosphorylation sites) at 1×10¹² vg/mL, and (3) AAV2/7-GFAP-NURR1-T2A-FOXA2 at 1×10¹² vg/mL — is delivered via stereotaxic co-injection (AP −3.1 mm, ML ±1.2 mm, DV −4.6 mm; 2 μL total volume) into the substantia nigra pars compacta (SNc) of 12-week-old male Aldh1l1-CreERT2;R26-tdTomato C57BL/6 mice at 1 week post-unilateral 6-OHDA lesion (tamoxifen-induced 2 weeks pre-lesion),

THEN at 12 weeks post-injection, ≥15% of tdTomato⁺ astrocyte-lineage cells in the ipsilateral SNc will be tdTomato⁺/TH⁺/NeuN⁺ triple-positive (vs. ≤2% in ASCL1-WT + PTBP1-KD dual-AAV controls and 0% in PTBP1-KD-alone controls verified by Aldh1l1-CreERT2 tracing), with concurrent striatal dopamine restoration to ≥50% of the intact hemisphere by HPLC-electrochemical detection and ≥60% reduction in amphetamine-induced ipsilateral rotational asymmetry versus lesion-only controls,

BECAUSE the following mechanistic cascade operates synergistically:

1. PTBP1 depletion by CasRx removes splicing suppression of neuronal transcripts: PTBP1 acts as a global repressor of the neuronal splicing program in astrocytes, silencing exons essential for synaptic function (e.g., Nrxn1, Snap25 neuronal isoforms). CasRx-mediated knockdown relieves this brake, licensing astrocytes to process pre-mRNA in a neuron-competent manner. However, PTBP1-KD alone achieves 0% genuine conversion by rigorous Aldh1l1-CreERT2 lineage tracing, confirming it is necessary but wholly insufficient for astrocyte-to-neuron fate change — as established by the Wang et al. Cell 2021 rebuttal described in the evidence set narrative.

2. Phospho-site-deficient ASCL1-SA6 provides maximally active proneural transcription factor activity that WT ASCL1 cannot sustain: In reactive SNc astrocytes, CDK1/2/4/5 kinases phosphorylate ASCL1 at six serine residues, targeting it for proteasomal degradation and blocking DNA binding. The phospho-dead SA6 mutant is resistant to this inactivation, sustaining chromatin remodeling at proneural E-box targets (including Dll1, Dcx, NeuroD1) for the weeks required to complete fate conversion. Critically, (Ascl1SA6 combined with Bcl2 achieved robust astrocyte-to-parvalbumin-interneuron conversion confirmed by Aldh1l1-CreERT2/RCE:loxP lineage tracing in cortex, whereas WT Ascl1 failed under identical conditions)[https://doi.org/10.1101/2023.11.03.565289]. The same CDK-mediated inactivation mechanism operates in SNc reactive astrocytes, where injury-induced CDK activity would neutralize any WT ASCL1 delivered by prior approaches — explaining why WT-ASCL1-based strategies stall at immature non-dopaminergic fates.

3. PTBP1-KD and ASCL1-SA6 act on orthogonal molecular nodes and together lower the epigenetic barrier below the threshold for genuine fate transition [SPECULATIVE]: PTBP1 depletion removes ...

SENS category: RepleniSENS

Key references: • doi.org/10.1101/2023.11.03.565289]. • doi.org/10.1101/2023.10.29.560483]. • doi.org/10.1101/2023.11.03.565289], • doi.org/10.1101/2023.11.03.565289] • doi.org/10.1101/2023.10.29.560483],