Chloride Balancing Act: The ionic composition of the cytosol and intracellular organelles must be regulated in the face of ongoing membrane traffic into and out of the cell. Now, two papers address the consequences of a change in the transport phenotype of an intracellular Cl ⁻ transport protein from a coupled exchanger to a passive Cl ⁻ conductor (see the Perspective by Smith and Schwappach ). Novarino et al. (p. 1398 , published online 29 April) investigated the consequence of a knock-in of the uncoupled ClC-5 transporter into mouse. The knock-out mouse of this endosomal kidney transporter has a severe endocytic phenotype believed to be due to a defect in vesicular acidification. The current study shows a similarly impaired endocytic phenotype for the uncoupled mutant, but the acidification of endosomes was unaffected. Weinert et al. (p. 1401 , published online 29 April) used a similar strategy to investigate the consequence of the equivalent mutation in the lysosomal transporter ClC-7, which is highly expressed in the resorption lacuna of osteoclasts and whose knock-out in mice produces lysosomal storage disease and severe osteopetrosis. A similar (though less severe) phenotype was observed in the knock-in mice containing the uncoupled ClC-7, indicating that coupled transport plays a critical role in lysosomes.

Neurotransmission relies on the calcium-triggered exocytic fusion of non-peptide neurotransmitter-containing small synaptic vesicles (SVs) with the presynaptic membrane at active zones (AZs) followed by compensatory endocytic retrieval of SV membranes. Here, we study the diffusional fate of newly exocytosed SV proteins in hippocampal neurons by high-resolution time-lapse imaging. Newly exocytosed SV proteins rapidly disperse within the first seconds post fusion until confined within the presynaptic bouton. Rapid diffusional spread and confinement is followed by slow reclustering of SV proteins at the periactive endocytic zone. Confinement within the presynaptic bouton is mediated in part by SV protein association with the clathrin-based endocytic machinery to limit diffusional spread of newly exocytosed SV proteins. These data suggest that diffusion, and axonal escape of newly exocytosed vesicle proteins, are counteracted by the clathrin-based endocytic machinery together with a presynaptic diffusion barrier.

Neutrophils, the most abundant and biotoxic immune cells, extrude nuclear DNA into the extracellular space to maintain homeostasis. Termed neutrophil extracellular traps (NETs), these protein-modified and decondensed extracellular DNA scaffolds control infection and are involved in coagulation, autoimmunity and cancer ^1,2 . Here we show how myeloperoxidase (MPO), a highly expressed neutrophil protein, disassembles nucleosomes, thereby facilitating NET formation, yet also binds stably to NETs extracellularly. We describe how the oligomeric status of MPO governs both outcomes. MPO dimers interact with nucleosomal DNA using one protomer and concurrently dock into the nucleosome acidic patch with the other protomer. As a consequence, dimeric MPO displaces DNA from the core complex, culminating in nucleosome disassembly. On the other hand, MPO monomers stably interact with the nucleosome acidic patch without making concomitant DNA contacts, explaining how monomeric MPO binds to and licences NETs to confer hypohalous acid production in the extracellular space ³ . Our data demonstrate that the binding of MPO to chromatin is governed by specific molecular interactions that transform chromatin into a non-replicative, non-encoding state that offers new biological functions in a cell-free manner. We propose that MPO is, to our knowledge, the first member of a class of proteins that convert chromatin into an immune effector.

All synapses require fusion-competent vesicles and coordinated Ca²⁺-secretion coupling for neurotransmission, yet functional and anatomical properties are diverse across different synapse types. We show that the presynaptic protein RIM-BP2 has diversified functions in neurotransmitter release at different central murine synapses and thus contributes to synaptic diversity. At hippocampal pyramidal CA3-CA1 synapses, RIM-BP2 loss has a mild effect on neurotransmitter release, by only regulating Ca²⁺-secretion coupling. However, at hippocampal mossy fiber synapses, RIM-BP2 has a substantial impact on neurotransmitter release by promoting vesicle docking/priming and vesicular release probability via stabilization of Munc13-1 at the active zone. We suggest that differences in the active zone organization may dictate the role a protein plays in synaptic transmission and that differences in active zone architecture is a major determinant factor in the functional diversity of synapses.