Age‑dependent mitochondrial ROS in corticostriatal axons disrupts D1 receptor‑ERK1/2 coupling, shifting basal ganglia output toward indirect pathway dominance before any detectable volumetric loss.

Mechanistic Rationale

Normal aging elevates superoxide production in presynaptic terminals that release glutamate onto dorsostriatal spiny neurons. This oxidative milieu modifies cysteine residues on the D1 receptor and its associated scaffolding protein Homer1, increasing the affinity of the striatal‑enriched phosphatase STEP for the receptor complex. STEP then dephosphorylates ERK1/2, breaking the dopamine‑D1‑ERK signaling cascade that normally strengthens action sequences in dSPNs and supports habit formation. Consequently, indirect pathway iSPNs, which retain D2‑mediated ERK activation, gain relative influence, biasing circuit output toward stereotyped behaviors even when dopaminergic tone is only modestly reduced. This mechanism explains why functional changes precede structural atrophy: synaptic redox alterations impair signaling cascades without causing immediate neuronal loss.

Testable Predictions

1. Biochemical signature – In aged mice expressing human A53T α‑synuclein, axonal mitochondrial ROS (measured with MitoSOX) will correlate inversely with pERK1/2 levels in dSPNs but not in iSPNs, and this correlation will be abolished by STEP knockdown.
2. Pharmacological rescue – Chronic treatment with the mitochondria‑targeted antioxidant MitoQ will restore D1‑ERK coupling, normalize D1 receptor binding PET signal, and reduce habit‑learning scores in a sequential lever‑press task, without altering striatal volume on high‑resolution MRI.
3. Genetic validation – Conditional overexpression of SOD2 specifically in corticostriatal axons will prevent ROS‑dependent STEP activation, preserve D1‑ERK signaling, and delay the emergence of habit bias in the same model, while leaving indirect pathway ERK activity unchanged.
4. Imaging biomarker – Multivariate patterns of basal ganglia‑thalamo‑cortical volume derived from MRI will remain indistinguishable between treated and untreated groups at early time points, yet diffusion‑weighted imaging will show preserved axonal integrity in the corticospinal tract only in the antioxidant or SOD2‑overexpression cohorts.

Experimental Approach

Use longitudinal cohorts of 12‑month‑old A53T α‑synuclein transgenic mice. Assign to four groups: vehicle, MitoQ (30 mg/kg/day), axonal‑SOD2 AAV vector, and scrambled AAV control. At 3, 6, and 9 months post‑injection, acquire PET with [^11C]SCH23390 (D1) and [^11C]raclopride (D2), perform ex‑vivo Western blot for pERK1/2 and STEP activity in microdissected dorsostriatal dSPN and iSPN fractions, run a probabilistic habit‑learning assay, and obtain 7 T MRI for volumetric and diffusion analysis.

Falsifiability

If MitoQ or axonal SOD2 fails to improve D1‑ERK coupling or habit performance despite reducing axonal ROS, or if STEP manipulation does not rescue the signaling deficit, the hypothesis would be refuted. Conversely, demonstration that restoring axonal redox balance uncouples the D1‑ERK deficit and normalizes behavior before any volumetric change would support the proposed mechanism and highlight a pre‑atrophy therapeutic window.