The Recycling Endosomal (Na+, K+)/H+ Exchanger NHE6/SLC9A6 Facilitates Signal Transduction by Shuttling Cyclin-Dependent Kinase 5 to the Plasma Membrane.

AIM: The alkali cation/proton exchanger NHE6/SLC9A6 regulates luminal pH homeostasis and trafficking of recycling endosomes in most tissues, especially neurons. Loss-of-function mutations in NHE6 cause Christianson Syndrome, an X-linked neurodevelopmental and neurodegenerative disorder; however, the underlying molecular and cellular mechanisms remain unclear. Here, we describe a new role for NHE6 as a scaffolding platform for recruiting and delivering signaling molecules to the plasma membrane.

METHODS: The yeast two-hybrid system was used to screen a human brain cDNA library for proteins that bind to the cytoplasmic C-terminus of NHE6.

RESULTS: Cyclin-dependent kinase 5 (CDK5) was identified as a putative interacting partner. CDK5 is widely expressed and phosphorylates diverse proteins involved in vital processes, including receptor signaling, cytoskeletal organization, endocytosis, exocytosis, and apoptosis. Formation of a NHE6/CDK5 complex was confirmed by biochemical assays and microscopy using Chinese hamster ovary AP-1 and human neuroblastoma SH-SY5Y cells. CDK5, in a complex with its activator subunit p35/CDK5R1, did not directly phosphorylate or regulate the membrane trafficking of NHE6. By contrast, NHE6 expression enhanced the localization of CDK5 and p35 to endosomal- and plasmalemmal-enriched membrane fractions and elevated cell surface accumulation of the CDK5-regulated transient receptor potential V1 (TRPV1) cation channel.

CONCLUSIONS: These data indicate that NHE6, aside from its main pH-regulatory function, can act concomitantly as a scaffold for recruitment of CDK5/p35 to endosomes and the plasma membrane where the kinase is now primed to activate neighboring effectors important for cell function.