Late endosome transport by RILP-RAB7A promotes dendrite arborization

Directional dendritic transport of late endosomes retrogradely towards the soma is required for fusion with lysosomes and for degradation in the soma. Both dendritic motility of late endosomes and somatic degradation require RAB7A. Similarly, interference with dynein function reduces motility of late endosomes and results in degradative failure. Blocking dynein function also impairs normal dendrite growth, suggesting that motility of late endosomes and/or lysosomes might be required for dendrite growth. RAB7A and dynein are mechanistically linked via RILP which is a dynein-interacting RAB7A effector. RILP also binds the late endosome-lysosome fusion tether HOPS. In non-neuronal cells, downregulation of RILP leads to impaired degradation due to deficiencies in late endosome transport and fusion defects with lysosomes. In this work, we express a separation-of-function mutant of RAB7A (RAB7A-L8A) incapable of RILP binding. Based on the results in non-neuronal cells, we hypothesized that both endosome motility and degradation in neurons depended on RILP. Our data in cultured rat and mouse hippocampal neurons of both sexes suggest that endogenous RILP is a functional RAB7A-dependent dynein adaptor for late endosome motility in dendrites. Interestingly, it also promotes endosome carrier formation. As a consequence of late endosome transport inhibition, degradative cargos are not cleared normally from dendrites in RAB7A-L8A. Surprisingly, lysosomal fusion and somatic degradation do not require RAB7A-RILP interactions. Despite the normal degradation, dendrite arborization is impaired in RAB7A-L8A expressing neurons, demonstrating that dendrite morphology defects are separable from degradation blockade. This indicates that normal dendrite growth/maintenance is dependent on sustained RAB7A/RILP-dependent LE transport.