Postsynaptic remodeling and F-actin dysregulation in the Dctn1Δ2-4 mouse model of motor neuron disease

The microtubule motor dynein and its co-adaptor dynactin govern organelle positioning within highly polarized neurons. The dynactin subunit p150Glued, encoded by DCTN1, is associated with motor neuron disease. The p150Glued G59S mutation disrupts the microtubule-binding domain and drives selective motor neuron degeneration in patients and animal models. The p150Glued microtubule-binding domain mediates the initiation of retrograde transport at axonal terminals and stabilizes presynaptic sites at neuromuscular junctions. Nevertheless, the key functions of p150Glued and its microtubule-binding domain within postsynaptic compartments remain poorly defined. Using Dctn1Δ2-4 mice, in which exons 2-4 of Dctn1 encoding the microtubule-binding domain are deleted, we performed morphological, biochemical, and cellular physiological analyses, together with genetic manipulation and pharmacological intervention, to investigate how loss of p150Glued microtubule-binding function affects excitatory postsynaptic organization and neuronal vulnerability. We found that excitatory postsynaptic remodeling occurred before overt dendritic degeneration and motor neuron loss in Dctn1Δ2-4 mice. We further demonstrated that p150Glued microtubule-binding domain dysfunction induced early remodeling of excitatory postsynaptic densities, leading to increased postsynaptic accumulation of N-methyl- D-aspartate receptors, dysregulated Ca2+ homeostasis, and heightened susceptibility to glutamate-induced excitotoxicity. Mechanistically, p150Glued microtubule-binding domain dysfunction promoted the postsynaptic accumulation of endosomes and the Wiskott-Aldrich syndrome protein and SCAR homolog complex, resulting in excessive F-actin assembly and pathological enhancement of postsynaptic density architecture. Furthermore, these postsynaptic abnormalities were rescued by re-expression of wild-type p150Glued, but not the disease-associated G59S mutant. Pharmacological inhibition of N-methyl-D-aspartate receptors, in turn, attenuated excitotoxic injury and neurodegeneration. Our findings identify a previously unrecognized postsynaptic pathogenic mechanism in DCTN1-associated motor neuron disease. We propose that loss of p150Glued microtubule-binding function disrupts postsynaptic endosome positioning, leading to aberrant Wiskott-Aldrich syndrome protein and SCAR homolog-mediated F-actin remodeling, pathological enhancement of excitatory postsynaptic densities, excessive N-methyl-D-aspartate receptor targeting, and increased vulnerability to excitotoxicity.