Regulatory imbalance between LRRK2 kinase, PPM1H phosphatase, and Arf6 GTPase disrupts the axonal transport of autophagosomes

Gain-of-function mutations in the LRRK2 gene cause Parkinson’s disease, increasing phosphorylation of RAB GTPases through hyperactive kinase activity. We find that LRRK2-hyperphosphorylated RABs disrupt the axonal transport of autophagosomes by perturbing the coordinated regulation of cytoplasmic dynein and kinesin motors. In iPSC-derived human neurons, knock-in of the strongly-hyperactive LRRK2-p.R1441H mutation caused striking impairments in autophagosome transport, inducing frequent directional reversals and pauses. Knock-out of the opposing Protein Phosphatase 1H (PPM1H) phenocopied the effect of hyperactive LRRK2. Overexpression of ADP-ribosylation factor 6 (ARF6), a GTPase that acts as a switch for selective activation of dynein or kinesin, attenuated transport defects in both p.R1441H knock-in and PPM1H knock-out neurons. Together, these findings support a model where a regulatory imbalance between LRRK2-hyperphosphorylated RABs and ARF6 induces an unproductive “tug-of-war” between dynein and kinesin, disrupting processive autophagosome transport. This disruption may contribute to PD pathogenesis by impairing the essential homeostatic functions of axonal autophagy.