Endosome maturation is orchestrated by inside-out proton signaling through a Na+/H+ exchanger and pH-dependent Rab GTPase cycling.

Endosome maturation requires lumen acidification. Is progressive lumen acidification sensed by cytosolic-side molecules driving maturation? We show here that proton efflux through the endosomal Na⁺/H⁺ Exchanger (NHE6) activates the late endosome master regulator Rab7. Importantly, NHE6 is mutated in the childhood neurologic disorder Christianson Syndrome. We demonstrate that NHE6 interacts with the Rab7 GTPase-activating protein (GAP) TBC1D5 in a complex with Rab7 on the late endosome. This interaction and proton efflux are both required for Rab7 activation. TBC1D5 is potently inactivated with decreasing pH. A conserved histidine in the TBC1D5 GAP domain mediates pH-dependence. Furthermore, we show that neurons from mice engineered with a selective defect in NHE6 proton efflux exhibit blocked endosome maturation and disrupted Rab7 GTP-GDP cycling. In addition, knock-down of TBC1D5, thereby reducing Rab7 GAP activity, in NHE6 mutant neurons rescues Rab7 GTP-GDP cycling and endosome maturation. Finally, we present a biophysical model of proton signaling through acidic pH microdomains within the NHE6-TBC1D5-Rab7 protein complex upon endosome acidification. In conclusion, our studies provide evidence supporting a mechanism involving "inside-out" proton signaling, whereby lumen acidification drives endosome maturation through pH-dependent Rab GTPase cycling. Failure in this mechanism may have broad impact in neurodegenerative disease.