Disruption of Golgi markers by two RILP-directed shRNAs in neurons: A new role for RILP or a neuron-specific off-target phenotype?

In neurons, degradation of dendritic cargos requires RAB7 and dynein-mediated retrograde transport to somatic lysosomes. To test if the dynein adapter RAB-interacting lysosomal protein (RILP) mediated the recruitment of dynein to late endosomes for retrograde transport in dendrites, we obtained several knockdown reagents previously validated in non-neuronal cells. Striking endosomal phenotypes elicited by one shRILP plasmid were not reproduced by another one. Furthermore, we discovered a profound depletion of Golgi/TGN markers for both shRILP plasmids. This Golgi disruption was only observed in neurons and could not be rescued by re-expression of RILP. This Golgi phenotype was also not found in neurons treated with siRILP or gRILP/Cas9. Lastly, we tested if a different RAB protein that interacts with RILP, namely the Golgi-associated RAB34, might be responsible for the loss of Golgi markers. Expression of a dominant-negative RAB34 did indeed cause changes in Golgi staining in a small subset of neurons but manifested as fragmentation rather than loss of staining. Unlike in non-neuronal cells, interference with RAB34 did not cause dispersal of lysosomes in neurons. Based on multiple lines of experimentation, we conclude that the neuronal Golgi phenotype observed with shRILP is likely off-target in this cell type specifically. Any observed disruptions of endosomal trafficking caused by shRILP in neurons might thus be downstream of Golgi disruption. It would be interesting to identify the actual target for this neuronal Golgi phenotype. Cell type-specific off-target phenotypes therefore likely occur in neurons, necessitating revalidation of reagents that were previously validated in other cell types.