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Acknowledgements NGS experiments were performed in the CNIC Genomics Unit (Centro Nacional de Investigaciones Cardiovasculares, Madrid, Spain) and analyzed by the CNIC Bioinformatics Unit. We thank Dr. Miguel Vicente‐Manzanares for critical reading of the manuscript and Dr. Henar Suárez and Dr. María Yañez‐Mó for their help with SEC experiments. This manuscript was funded by grants SAF 2017‐82886‐R (FS‐M) from the Spanish Ministry of Economy and Competitiveness; CAM (S2017/BMD‐3671‐INFLAMUNE‐CM) from the Comunidad de Madrid (FS‐M); CIBERCV (CB16/11/00272), BIOIMID PIE13/041 from the Instituto de Salud Carlos III and from the Fundación La Marató TV3 (grant 122/C/2015). The current research has received funding from “la Caixa” Foundation under the project code HR17‐00016. VGY is supported by the AECC foundation. A.R.R. is supported by CNIC funding. This project was funded by the Spanish Ministerio de Ciencia, Innovacion y Universidades SAF2016‐75511‐R, and La Caixa Health Research Program HR17‐00247 grant to A.R.R. Grants from Ramón Areces Foundation “Ciencias de la Vida y de la Salud” (XIX Concurso‐2018) and from Ayuda Fundación BBVA y Equipo de Investigación Científica (BIOMEDICINA‐2018) (to FSM). The CNIC is supported by the Ministerio de Ciencia, Innovacion y Universidades and the Pro‐CNIC Foundation, and is a Severo Ochoa Center of Excellence (SEV‐2015‐0505)., Intercellular communication orchestrates effective immune responses against disease-causing agents. Extracellular vesicles (EVs) are potent mediators of cell-cell communication. EVs carry bioactive molecules, including microRNAs, which modulate gene expression and function in the recipient cell. Here, we show that formation of cognate primary T-B lymphocyte immune contacts promotes transfer of a very restricted set of T-cell EV-microRNAs (mmu-miR20-a-5p, mmu-miR-25-3p, and mmu-miR-155-3p) to the B cell. Transferred EV-microRNAs target key genes that control B-cell function, including pro-apoptotic BIM and the cell cycle regulator PTEN. EV-microRNAs transferred during T-B cognate interactions also promote survival, proliferation, and antibody class switching. Using mouse chimeras with Rab27KO EV-deficient T cells, we demonstrate that the transfer of small EVs is required for germinal center reaction and antibody production in vivo, revealing a mechanism that controls B-cell responses via the transfer of EV-microRNAs of T-cell origin. These findings also provide mechanistic insight into the Griscelli syndrome, associated with a mutation in the Rab27a gene, and might explain antibody defects observed in this pathogenesis and other immune-related and inflammatory disorders., Depto. de Biología Celular, Fac. de Ciencias Biológicas, TRUE, pub

Tissue injury and inflammation may result in chronic pain, a severe debilitating disease that is associated with great impairment of quality of life. An increasing body of evidence indicates that members of the Rab family of small GTPases contribute to pain processing; however, their specific functions remain poorly understood. Here, we found using immunofluorescence staining and in situ hybridization that the small GTPase Rab27a is highly expressed in sensory neurons and in the superficial dorsal horn of the spinal cord of mice. Rab27a mutant mice, which carry a single-nucleotide missense mutation of Rab27a leading to the expression of a nonfunctional protein, show reduced mechanical hyperalgesia and spontaneous pain behavior in inflammatory pain models, while their responses to acute noxious mechanical and thermal stimuli is not affected. Our study uncovers a previously unrecognized function of Rab27a in the processing of persistent inflammatory pain in mice.

Rab GTPases are compartment-specific molecular switches that regulate intracellular vesicular transport in eukaryotes. GDP/GTP exchange factors (GEFs) control Rab activation, and current models propose that localised and regulated GEF activity is important in targeting Rabs to specific membranes. Here, we investigated the mechanism of GEF function using the Rab27a GEF, Rab3GEP (also known as MADD), in melanocytes as a model. We show that Rab3GEP-deficient melanocytes (melan-R3GKO) manifest partial disruption of melanosome dispersion, a read-out of Rab27a activation and targeting. Using rescue of melanosome dispersion in melan-R3GKO cells and effector pull-down approaches we show that the DENN domain of Rab3GEP (conserved among RabGEFs) is necessary, but insufficient, for its cellular function and GEF activity. Finally, using a mitochondrial re-targeting strategy, we show that Rab3GEP can target Rab27a to specific membranes in a GEF-dependent manner. We conclude that Rab3GEP facilitates the activation and targeting of Rab27a to specific membranes, but that it differs from other DENN-containing RabGEFs in requiring DENN and non-DENN elements for both of these activities and by lacking compartment-specific localisation.