Hypothesis

The protective role of alpha-synuclein aggregation depends on the ability of a given neuron to sequester toxic oligomers into large, inert Lewy bodies without compromising the pool of soluble alpha-synuclein required for normal synaptic function. We propose that striatal D2-medium spiny neurons (MSNs) possess a lower intrinsic aggregation capacity than D1-MSNs, making them vulnerable to oligomeric toxicity and contributing to the age‑related D1/D2 imbalance. Enhancing the aggregation machinery specifically in D2-MSNs will shift the equilibrium toward large Lewy bodies, thereby reducing toxic oligomers while preserving sufficient soluble alpha‑synuclein for its physiological roles.

Novel mechanistic insight: Differential expression of the HSP40 co‑chaperone DNAJB6 (which promotes amyloid‑like nucleation and suppresses oligomer formation) underlies the subtype disparity. D1-MSNs express higher basal DNAJB6, facilitating efficient sequestration into large aggregates, whereas D2-MSNs exhibit reduced DNAJB6, leading to accumulation of soluble oligomers and loss‑of‑function. Restoring DNAJB6 levels in D2-MSNs should therefore act as a proteostatic ‘sink’ that converts dangerous oligomers into benign inclusions.

Testable predictions:

1. Viral overexpression of DNAJB6 in D2-MSNs of aged mice will increase the proportion of alpha‑synuclein present in Sarkosyl‑insoluble, high‑molecular‑weight fractions (indicative of large aggregates) and decrease Sarkosyl‑soluble oligomeric species, as measured by biochemical fractionation and conformation‑specific antibodies.
2. This shift will correlate with a rescue of the D1/D2 firing ratio in basal ganglia output nuclei, measured by in vivo electrophysiology or fiber‑photometry of pathway‑specific calcium signals.
3. Behaviorally, aged mice with D2‑MSN DNAJB6 overexpression will show improved performance on habit‑formation tasks (e.g., lever‑press sequencing) that are impaired by D1/D2 imbalance, without compromising goal‑directed learning.
4. Conversely, CRISPR‑mediated knockdown of DNAJB6 in D1-MSNs will exacerbate oligomer accumulation, increase cytotoxicity, and worsen the D1/D2 imbalance, providing a falsifying counterpart.

Experimental approach: Use Cre‑dependent AAV vectors in D1‑Cre or D2‑Cre mouse lines to manipulate DNAJB6 levels; assess alpha‑synuclein species by western blot, filter trap, and immunofluorescence for Lewy body morphology; monitor neuronal survival with NeuN staining; evaluate circuit dynamics with D1‑ and D2‑specific GCaMP recordings; test habit learning in a probabilistic reinforcement‑learning paradigm.

If increasing aggregation capacity in vulnerable D2-MSNs restores balance, it would reframe therapeutic strategies: rather than blocking aggregation globally, enhancing selective sequestration may protect neurons while preserving essential protein function.