Exosomes are vehicles for the stress-regulated secretion of histones

Histones are conserved nuclear proteins that function in the regulation of chromatin structure and gene expression. Interestingly, extracellular histones populate biofluids from healthy individuals and when elevated may contribute to various acute and chronic diseases. It is unknown if histones are secreted via a controlled pathway, or merely released from damaged or dying cells. We analysed cell culture models under normal and stressed conditions to identify pathways of histone secretion. We report that core and linker histones localize to extracellular vehicles (EVs) and are secreted via the multivesicular body/exosome pathway. Most histone localized to the EV membrane. Upregulation of histone secretion via EVs occurred following cellular stress, accompanied by enhanced vesicle secretion and a shift towards a population of smaller EVs. Using genetic or pharmacological intervention, we found that multiple pathways affected the localization of histones to EVs. Surface histones colocalized most frequently with EVs carrying the tetraspanin CD63. Unexpectedly, EV-associated histones lacked extensive post-translational modifications compared to nuclear histones. We show for the first time that membrane-associated histones are actively secreted from intact cells via EVs/exosomes. This fundamental discovery provides support for further investigation of the biological activity of exosome-associated histones and their role in disease.