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The release of preformed mediators from immune cells is through a process described as exocytosis. In mast cells, exocytosis is regulated by several coordinated intracellular signaling pathways. Here, we investigated the role of the hematopoietic-specific Rho GTPase, Rac2, and the ubiquitously expressed Rac1, in controlling mast cell exocytosis. These two isoforms showed equivalent levels of expression in mouse BMMCs. Although Rac1 and Rac2 share 92% sequence identity, they were not functionally redundant, as Rac2−/− BMMCs were defective in exocytosis, even though Rac1 levels were unaffected. Antigen-stimulated WT mast cells underwent a series of morphological transitions: initial flattening, followed by actin-mediated peripheral membrane ruffling and calcium influx, which preceded exocytosis. Whereas membrane ruffling was unaffected in Rac2−/− BMMCs, calcium influx was decreased significantly. Calcium influx was studied further by examining SOCE. In Rac2−/− BMMCs, the activation of PLCγ1 and calcium release from intracellular stores occurred normally; however, activation of plasma membrane calcium channels was defective, shown by the lack of extracellular calcium influx and a reduction of YFP-STIM1 puncta at the plasma membrane. Additionally, we used the small molecule Rac inhibitor, EHT 1864, to target Rac signaling acutely in WT BMMCs. EHT 1864 blocked exocytosis and membrane ruffling completely in conjunction with exocytosis. Our findings suggest that antigen-stimulated membrane ruffling in mast cells is a Rac1-mediated process, as this persisted in the absence of Rac2. Therefore, we define distinct modes of Rac-regulated mast cell exocytosis: Rac2-mediated calcium influx and Rac1-mediated membrane ruffling.

Mast cells are tissue-resident immune cells that produce potent proinflammatory mediators, which are stored in cytoplasmic granules. Stimulation triggers degranulation, a process that mobilizes granules to dock and fuse to the plasma membrane, releasing mediators. Mast cell degranulation has an important role in immunity but can also intensify inflammation and contribute to allergic disorders. Hence, it is important to understand signaling pathways that regulate mast cell degranulation. Here, we examined the role of Rho proteins in regulating mast cell activation leading to degranulation. RBL-2H3 cells and bone marrow–derived mast cells (BMMCs) were stimulated through aggregation of FcɛRI receptors. Stimulated cells showed a large increase in the levels of activated Rac and, to a lesser extent, RhoA. Drugs were used to acutely inhibit the function of specific Rho proteins. The Rac inhibitor EHT-1864 and the RhoA inhibitor rhosin inhibited degranulation. Microscopic characterization showed that, upon stimulation, RBL-2H3 cells formed surface ridges that grew into large protrusions reminiscent of circular dorsal ruffles, which flattened into large lamellipodia. LysoTracker-labeled cells showed granules stream into peripheral protrusions. EHT-1864 reduced granule motility, whereas rhosin increased motility; both drugs affected the formation of peripheral protrusions. These results showed that, in response to stimuli, Rho proteins control discrete cytoskeletal remodeling processes that are needed for granule exocytosis. Rac is required to stimulate the remodeling of mast cells, triggering actin-mediated flattening of the cell periphery to create an active degranulation zone, whereas RhoA controls the streaming of highly motile granules into the active zone.

First longitudinal studies in prisoners point to improvements of depressive symptoms during imprisonment. The aim of the present study was to assess the course of major depressive disorder during imprisonment and to identify factors influencing remission.Prisoners with major depressive disorder in a sample of consecutive admissions to the penal justice system in Santiago de Chile were reassessed after one year of imprisonment. Psychiatric diagnoses were established using the Mini-International Neuropsychiatric Interview; psychological symptoms were assessed with the Symptom-Check-List 90 Revised (SCL-90-R). Mean symptom scores were compared at baseline and follow-up using Student's t-test. Odds ratios (OR) of comorbid disorders and socio-demographic factors at baseline to predict depression at follow-up were calculated.N=79 out of 80 inmates (99%) with major depression at baseline were included. Thirty-five prisoners (44%) had major depression at follow-up. The mean global severity score and all mean subscale scores of the SCL-90-R improved. High suicide risk was present in 37 prisoners (47%) at admission and in 11 (14%) at follow-up. The comorbid diagnosis of PTSD (OR 6.3; p0.001) at admission and having been previously imprisoned (OR 2.5; p=0.05) predicted major depressive disorder at follow-up.The study could not account for temporary improvements between the assessments.In spite of important symptom improvements, only about half of the prisoners with major depression at admission remit after one year of imprisonment. New interventions should target people with major depression and comorbid PTSD at admission.

RhoGDIs (Rho GDP-dissociation inhibitors) are the natural inhibitors of Rho GTPases. They interfere with Rho protein function by either blocking upstream activation or association with downstream signalling molecules. RhoGDIs can also extract membrane-bound Rho GTPases to form soluble cytosolic complexes. We have shown previously that purified yeast RhoGDI Rdi1p, can inhibit vacuole membrane fusion in vitro. In the present paper we functionally dissect Rdi1p to discover its mode of regulating membrane fusion. Overexpression of Rdi1p in vivo profoundly affected cell morphology including increased actin patches in mother cells indicative of polarity defects, delayed ALP (alkaline phosphatase) sorting and the presence of highly fragmented vacuoles indicative of membrane fusion defects. These defects were not caused by the loss of typical transport and fusion proteins, but rather were linked to the reduction of membrane localization and activation of Cdc42p and Rho1p. Subcellular fractionation showed that Rdi1p is predominantly a cytosolic monomer, free of bound Rho GTPases. Overexpression of endogenous Rdi1p, or the addition of exogenous Rdi1p, generated stable cytosolic complexes. Rdi1p structure–function analysis showed that membrane association via the C-terminal β-sheet domain was required for the functional inhibition of membrane fusion. Furthermore, Rdi1p inhibited membrane fusion through the binding of Rho GTPases independent from its extraction activity.