Axonal Lysosomal Assays for Characterizing the Effects of LRRK2 G2019S

Simple Summary Axons are highly elongated extensions of neurons that transmit electrical impulses to their connecting targets and are integral for neuronal function. In neurodegenerative diseases such as Parkinson's disease, axons degenerate early in the disease course, breaking essential connections, leading to the development of clinical phenotypes over time. Thus, developing a better understanding of axonal pathology is crucial. For this reason, we used patient-derived induced pluripotent stem cells to generate neurons which were then used to develop assays to characterize how neurodegenerative diseases such as Parkinson's disease might affect axons. We show that LRRK2 G2019S, which is one of the most common known mutations causing Parkinson's disease, subtly affects axonal function and the injury response. Our assays could be used in the future to better understand axonal degeneration and test potential therapeutics for their ability to protect axons against degeneration.Abstract The degeneration of axon terminals before the soma, referred to as "dying back", is a feature of Parkinson's disease (PD). Axonal assays are needed to model early PD pathogenesis as well as identify protective therapeutics. We hypothesized that defects in axon lysosomal trafficking as well as injury repair might be important contributing factors to "dying back" pathology in PD. Since primary human PD neurons are inaccessible, we developed assays to quantify axonal trafficking and injury repair using induced pluripotent stem cell (iPSC)-derived neurons with LRRK2 G2019S, which is one of the most common known PD mutations, and isogenic controls. We observed a subtle axonal trafficking phenotype that was partially rescued by a LRRK2 inhibitor. Mutant LRRK2 neurons showed increased phosphorylated Rab10-positive lysosomes, and lysosomal membrane damage increased LRRK2-dependent Rab10 phosphorylation. Neurons with mutant LRRK2 showed a transient increase in lysosomes at axotomy injury sites. This was a pilot study that used two patient-derived lines to develop its methodology; we observed subtle phenotypes that might correlate with heterogeneity in LRRK2-PD patients. Further analysis using additional iPSC lines is needed. Therefore, our axonal lysosomal assays can potentially be used to characterize early PD pathogenesis and test possible therapeutics.