Membrane-associated Rhes-Slc4a7 complex orchestrates tunneling nanotube formation and mutant Huntingtin spread

Tunneling nanotubes (TNTs) are membranous structures that mediate intercellular transfer of proteins, including the pathogenic mutant Huntingtin (mHTT) protein in Huntington disease (HD). We previously identified the ras homolog enriched in the striatum (Rhes) as a key regulator of TNT formation and mHTT transmission; however, the molecular components underlying this process remained unknown. Here, using unbiased liquid chromatography-tandem mass spectrometry analysis of membrane-associated Rhes complexes, we identify Slc4a7 (solute carrier family 4 member 7), an intracellular pH sensor, as a top membrane-binding partner of Rhes. Functional studies revealed that small interfering RNA-mediated depletion or pharmacological inhibition of Slc4a7 substantially reduced Rhes-induced TNT formation and suppressed mHTT intercellular transfer. Mechanistically, Rhes directly interacts with Slc4a7 through both its amino- and carboxyl-terminal domains and modulates intracellular pH to facilitate TNT formation. This interaction does not depend on the transporter activity of Slc4a7. However, inhibition of Rhes farnesylation-a lipid modification that anchors Rhes to the membrane-disrupts its binding to Slc4a7 and abolishes TNT formation. Slc4a7 knock-out mice showed markedly reduced cell-to-cell transmission of mHTT in the striatum in vivo. Together, these findings uncover a previously unrecognized Rhes-Slc4a7 signaling axis critical for TNT-mediated mHTT transmission and highlight Slc4a7 as a potential therapeutic target to limit disease spread in HD.