Biomolecular tracking by FIRESCAPE reveals distinct modes of clearance, damage induction and cellular uptake for extracellular histone H3

In attempting to observe the behaviour of a protein of interest in vivo, akin to other observer effects, current techniques require modifications or interventions that inherently alter the protein or its host, leaving one uncertain as to whether natural behaviour remains unperturbed. The study of chromatin has mostly been restricted to defining its function in the nucleus, where histone proteins fulfil vital roles in packaging genomic DNA and regulating transcription. However, chromatin components can be released into the extracellular space, either intentionally via cellular secretion or during disease-induced cell death. These extracellular chromatin components, depending on the context, can consist of: free histones, free DNA, intact nucleosomes (histone octamers wrapped by DNA), or heterogeneous, higher order structures such as neutrophil extracellular traps (NETs). They have been associated with diverse pathologies such as inflammation, cancer, and sepsis, and distinct toxic effects. However, there is a widely acknowledged lack of methods that distinguish between them and between their unique functions. Here, we now address protein observer effects to explore the fate and function of extracellular free histones by utilizing FIRESCAPE ([18F]-Fluorine Isotopic Radiolabeling Enabling Scanning of Clearance After Proteolytic Events), a novel radiolabeling concept that leverages the unique, high-sensitivity properties of the radioisotope fluorine-18, 18F, and residue-specific protein editing chemistry. By installing close and then true 18F-containing protein sidechain mimics site-specifically, FIRESCAPE enables the hierarchical in vivo scanning of the half-lives, proteolytic susceptibility and clearance of single residues in a protein of interest, and at microdoses far below toxic levels (low nanomole). These radioequivalent proteins bearing near-zero-size, zero-background labels now precisely reveal the strikingly distinct distribution, half-lives, damage-inducing abilities and accumulation of free extracellular histones in cellulo and in vivo compared to intact nucleosomes. Free extracellular histone H3 is rapidly cleared from circulation, mediated first by proteolysis of the histone tail. By contrast, direct injection of free histones vs nucleosomes into tissue that is unprotected by such proteolysis (brain), provokes a starkly different response; free histones exhibited limited diffusion and swiftly promoted damage both in cell culture and in vivo, whilst intact nucleosomes were essentially passive and benign. Remarkably, synthetic extracellular histone H3 was observed to enter cells and integrate into chromatin, indistinguishable from native H3 in both localization and post-translational modification (PTM) accumulation, yet paired with cellular and tissue damage. The exploratory studies described here now provide much needed clarity to the distinct fates and effects of extracellular histones vs nucleosomes, in particular the strongly damaging effects of free histones, their rapid uptake into cells, and an associated histone-specific proteolysis pathway via the removal of the histone tail.