A light at the end of the tunnel for Huntington’s disease treatment
New research reveals how tiny ‘tunnels’ allow mutant huntingtin protein to spread between neurons, changing our understanding of Huntington’s disease progression and offering a promising therapeutic target.
Sparking fresh hope for a Huntington’s disease treatment, a team led by scientists from Florida Atlantic University (FL, USA) has identified a novel cellular pathway that allows brain cells to pass toxic material, such as the protein responsible for Huntington’s, to their neighbors via tiny, tube-like structures. Crucially, the researchers also demonstrate that disrupting this process dramatically reduces the spread of the disease-causing protein in the brain, potentially paving the way for novel therapeutics.
Huntington’s disease is characterized by the spread of the mutant huntingtin protein (mHTT) between brain cells. This can occur via tunneling nanotubes (TNT) – thin, actin-based membranous structures that form cytoplasmic bridges between cells. The researchers previously identified a protein called Rhes as a key regulator of nanotube formation and mHTT transmission; however, the molecular mechanisms underlying this process remain unknown.
With hopes of changing that, the team performed unbiased liquid chromatography–tandem mass spectrometry (LC-MS/MS) analysis to identify the intracellular pH sensor Slc4a7 as a key membrane-binding partner of Rhes, responsible for regulating TNT formation and mHTT transmission.
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Initially, to assess the role of Rhes in TNT formation, they generated cell lines expressing green fluorescent protein (EGFP) or EGFP-Rhes using lentiviral vectors in mouse striatal neuronal cells. These were then used to establish stable EGFP or EGFP-Rhes expression in human SH-SY5Y neuroblastoma cells and Huntington’s disease human induced pluripotent stem cells. As suspected, GFP-Rhes strongly induced TNT-like structures.
They then went on to identify membrane-associated protein interactors of Rhes by transfecting striatal neuronal cells with EGFP, EGFP-Rhes or EGFP-Rhes C263S (a membrane binding–deficient mutant) and performing biochemical membrane fractionation. Fractions were subjected to immunoprecipitation to selectively isolate GFP-tagged proteins, which then underwent LC-MS/MS to uncover proteins interacting with membrane-associated Rhes.
This process identified 188 proteins, from which Slc4a7 emerged as a promising candidate. This was confirmed by siRNA (small interfering RNA) screening and knockdown studies, which revealed that inhibition of Slc4a7 substantially decreased Rhes-induced TNT formation and suppressed mHTT intercellular transfer.
Using various advanced protein-mapping techniques, the team was also able to describe the mechanism of this interaction. They discovered that Rhes directly interacts with Slc4a7 through its amino- and carboxyl-terminal domains and modulates intracellular pH to facilitate TNT formation.
Moreover, in vivo experiments in Slc4a7 knock-out mice revealed that cell-to-cell transmission of mHTT in the striatum – the part of the brain most affected by Huntington’s disease – is markedly reduced, suggesting Slc4a7 as a potential therapeutic target to limit disease spread.
“This work fundamentally changes how we think about disease progression in Huntington’s,” stated Srinivasa Subramaniam, senior author of the study.
“By learning how harmful proteins physically move from cell to cell, we gain powerful new leverage points for therapy,” added Randy Blakely, executive director of Florida Atlantic University’s Stiles-Nicholson Brain Institute, who was not involved in the study. “The idea that we could slow or even halt disease progression by blocking these microscopic tunnels opens an exciting frontier for treating not only Huntington’s disease, but a wide range of neurological disorders and cancers in the future.”