Hypothesis

Senescent astrocytes within the dorsal striatum secrete osteopontin (SPP1) that binds CD44 receptors on medium spiny neurons (MSNs), activating Src family kinases to bias D1‑ versus D2‑receptor-coupled signaling pathways. This senescent‑derived signaling stabilizes the excitation/inhibition balance required for the transition from goal‑directed to habitual behavior. Clearing these senescent astrocytes disrupts osteopontin‑CD44‑Src signaling, shifts D1/D2 receptor coupling toward excessive D2 dominance, and impairs habit formation without affecting acute motor performance.

Mechanistic Basis

Recent work shows senescent glial cells propagate senescence through paracrine signals such as CCL2 and MIF in substantia nigra circuits[1,2]. In the striatum, however, senescent astrocytes may adopt a distinct SASP enriched in extracellular matrix modulators. Osteopontin, a phosphorylated glycoprotein known to interact with CD44 and integrin receptors, can activate Src kinases that phosphorylate D1 receptors, enhancing their coupling to Gs/adenylyl cyclase while reducing D2‑Gi signaling[3]. This dual modulation creates a signaling window that promotes long‑term potentiation in D1‑MSN circuits critical for habit consolidation. Senolytic clearance removes this modulatory input, tipping the balance toward D2‑mediated indirect pathway dominance and resulting in behavioral inflexibility.

Testable Predictions

1. Aged mice (≥18 mo) will show increased SPP1 immunoreactivity co‑localized with glial fibrillary acidic protein (GFAP)+ astrocytes in the dorsal striatum compared with young controls.
2. Genetic or pharmacologic ablation of senescent astrocytes (e.g., p16‑INK4a‑driven inducible caspase‑9) will reduce striatal SPP1 levels by >50% and decrease Src‑phosphorylated D1 receptors without altering total D1/D2 receptor expression.
3. These mice will exhibit impaired performance on a sequential lever‑press habit task (increased sensitivity to outcome devaluation) while retaining normal rotarod and open‑lane locomotion.
4. Exogenous application of recombinant osteopontin to striatal slices from senescent‑astrocyte‑ablated mice will rescue D1‑receptor Src phosphorylation and restore habit‑learning performance.
5. Conversely, over‑expressing SPP1 in astrocytes of young mice will precociously enhance habit formation, shifting the balance toward D1‑MSN dominance.

Experimental Approach

• Use p16‑3MR mice to label and selectively eliminate senescent astrocytes via ganciclovir infusion into the dorsal striatum; validate ablation by flow cytometry for p16+/GFAP+ cells.
• Quantify SPP1 ELISA and Western blot for phospho‑Src (Y416) and phospho‑D1 receptor (Ser) in microdissected striatal tissue.
• Behavioral assessment: (a) accelerated rotarod for motor function; (b) two‑step sequential lever‑press training followed by outcome devaluation to measure habit versus goal‑directed control; (c) open‑field locomotion for baseline activity.
• Rescue experiments: intracerebral striatal delivery of recombinant SPP1 or CD44‑Fc fusion protein to block endogenous signaling.
• Statistical analysis: two‑way ANOVA (age × treatment) with post‑hoc Tukey; power analysis targeting n=12 per group to detect 20% effect size at α=0.05, β=0.2.

Falsifiability

If senescent astrocyte ablation does not alter striatal SPP1 levels, phospho‑D1/Src signaling, or habit‑learning performance—while still reducing SASP‑associated inflammatory markers—the hypothesis would be refuted. Similarly, if exogenous SPP1 fails to rescue habit deficits after senescent cell clearance, the proposed osteopontin‑CD44‑Src mechanism would be unsupported, indicating that senescent astrocytes influence behavior through alternative pathways.