Localized protein synthesis requires intracellular RNA transport, whereby mRNAs are trafficked within RNA granules — membraneless organelles that form through liquid–liquid phase separation. The mechanisms involved in this transport are poorly understood. Liao et al. now show that in mammalian cells, RNA granules are trafficked by ‘hitchhiking’ on lysosomes.

Credit: Getty/Malte Mueller

The authors established that in the human U2OS cell line and in primary rat cortical neurons RNA granules associate with the surface of lysosome-like organelles and traffic on them. They then identified annexin A11 (ANXA11) as a protein that interacts with both granules and lysosomes, and thus could serve as a tether between these organelles.

In U2OS cells ANXA11 partitioned to RNA granules in a manner dependent on the N-terminal low-complexity region, which mediates its liquid–liquid phase separation. The interaction with the lysosome, in turn, depended on the C-terminal annexin domains of ANXA11, which bind — in a calcium-dependent manner — to endolysosomal phosphoinositides.

ANXA11 was sufficient to drive association between RNA granules and lysosome-like vesicles in vitro. ANXA11 also co-localized with motile lysosome–RNA granule complexes in neurons, where it was required for unperturbed RNA granule hitchhiking on lysosomes and for efficient RNA delivery to the distal parts of the cell.

Mutations in the gene ANX11 are linked to neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). ALS-causing mutations interfered with phase separation dynamics of ANXA11 and reduced its ability to partition to RNA granules, and also impeded ANXA11 binding to lysosomal membranes. Accordingly, expression of ANXA11 mutants perturbed RNA granule–lysosome contacts, reduced the number of hitchhiking events and interfered with intracellular mRNA transport in neurons, both in culture and in zebrafish larvae.

ALS-causing mutations interfered with phase separation dynamics of ANXA11

In summary, efficient mRNA trafficking is supported by tethering of RNA granules to motile lysosomes through ANXA11, which is regulated by phase separation and calcium signalling. Deregulation of these mechanisms could contribute to neurodegeneration.