Syntaxin 3, but not syntaxin 4, is required for mast cell-regulated exocytosis, where it plays a primary role mediating compound exocytosis.

Mast cells (MCs) participate in allergy, inflammation, and defense against pathogens. They release multiple immune mediators via exocytosis, a process that requires SNARE proteins, including syntaxins (Stxs). The identity of the Stxs involved in MC exocytosis remains controversial. Here, we studied the roles of Stx3 and -4 in fully developed MCs from conditional knockout mice by electrophysiology and EM, and found that Stx3, and not Stx4, is crucial for MC exocytosis. The main defect seen in Stx3-deficient MCs was their inability to engage multigranular compound exocytosis, while leaving most single-vesicle fusion events intact. We used this defect to show that this form of exocytosis is not only required to accelerate MC degranulation but also essential to achieve full degranulation. The exocytic defect was severe but not absolute, indicating that an Stx other than Stx3 and -4 is also required for exocytosis in MCs. The removal of Stx3 affected only regulated exocytosis, leaving other MC effector responses intact, including the secretion of cytokines via constitutive exocytosis. Our in vivo model of passive systemic anaphylaxis showed that the residual exocytic function of Stx3-deficient MCs was sufficient to drive a full anaphylactic response in mice.

Munc13 proteins control regulated exocytosis in mast cells.

Mast cells (MCs) are involved in host defenses against pathogens and inflammation. Stimulated MCs release substances stored in their granules via regulated exocytosis. In other cell types, Munc13 (mammalian homolog of Caenorhabditis elegans uncoordinated gene 13) proteins play essential roles in regulated exocytosis. Here, we found that MCs express Munc13-2 and -4, and we studied their roles using global and conditional knock-out (KO) mice. In a model of systemic anaphylaxis, we found no difference between WT and Munc13-2 KO mice, but global and MC-specific Munc13-4 KO mice developed less hypothermia. This protection correlated with lower plasma histamine levels and with histological evidence of defective MC degranulation but not with changes in MC development, distribution, numbers, or morphology. In vitro assays revealed that the defective response in Munc13-4-deficient MCs was limited to regulated exocytosis, leaving other MC secretory effector responses intact. Single cell capacitance measurements in MCs from mouse mutants differing in Munc13-4 expression levels in their MCs revealed that as levels of Munc13-4 decrease, the rate of exocytosis declines first, and then the total amount of exocytosis decreases. A requirement for Munc13-2 in MC exocytosis was revealed only in the absence of Munc13-4. Electrophysiology and EM studies uncovered that the number of multigranular compound events (i.e. granule-to-granule homotypic fusion) was severely reduced in the absence of Munc13-4. We conclude that although Munc13-2 plays a minor role, Munc13-4 is essential for regulated exocytosis in MCs, and that this MC effector response is required for a full anaphylactic response.

Association of Hyperchloremia With Hospital Mortality in Critically Ill Septic Patients.

OBJECTIVES: Hyperchloremia is frequently observed in critically ill patients in the ICU. Our study aimed to examine the association of serum chloride (Cl) levels with hospital mortality in septic ICU patients. DESIGN: Retrospective cohort study. SETTING: Urban academic medical center ICU. PATIENTS: ICU adult patients with severe sepsis or septic shock who had Cl measured on ICU admission were included. Those with baseline estimated glomerular filtration rate less than 15 mL/min/1.73 m or chronic dialysis were excluded. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Of 1,940 patients included in the study, 615 patients (31.7%) had hyperchloremia (Cl ≥ 110 mEq/L) on ICU admission. All-cause hospital mortality was the dependent variable. Cl on ICU admission (Cl0), Cl at 72 hours (Cl72), and delta Cl (ΔCl = Cl72 - Cl0) were the independent variables. Those with Cl0 greater than or equal to 110 mEq/L were older and had higher cumulative fluid balance, base deficit, and Sequential Organ Failure Assessment scores. Multivariate analysis showed that higher Cl72 but not Cl0 was independently associated with hospital mortality in the subgroup of patients with hyperchloremia on ICU admission (adjusted odds ratio for Cl72 per 5 mEq/L increase = 1.27; 95% CI, 1.02-1.59; p = 0.03). For those who were hyperchloremic on ICU admission, every within-subject 5 mEq/L increment in Cl72 was independently associated with hospital mortality (adjusted odds ratio for ΔCl 5 mEq/L = 1.37; 95% CI, 1.11-1.69; p = 0.003). CONCLUSIONS: In critically ill septic patients manifesting hyperchloremia (Cl ≥ 110 mEq/L) on ICU admission, higher Cl levels and within-subject worsening hyperchloremia at 72 hours of ICU stay were associated with all-cause hospital mortality. These associations were independent of base deficit, cumulative fluid balance, acute kidney injury, and other critical illness parameters.

Association of de novo dipstick albuminuria with severe acute kidney injury in critically ill septic patients.

BACKGROUND: Acute kidney injury (AKI) occurs frequently in septic patients. Albuminuria may play a role as an early marker of septic AKI. The potential association between de novo dipstick albuminuria (DA) and septic AKI has not been examined. METHODS: We conducted a single-center observational cohort study of 423 critically ill septic patients without chronic kidney disease (CKD) or prior positive DA within 3 months before admission. The association between de novo DA within the first 24 h of presentation and AKI at 72 h was examined. RESULTS: AKI was identified in 268/423 (63%) patients and 20/423 (4.7%) required dialysis. De novo DA was associated with AKI (univariate OR 1.91; 95% CI 1.27-2.86, p = 0.002). The association persisted in a multivariate logistic regression model adjusted for demographics, baseline kidney function, comorbidities, critical illness parameters, and exposure to nephrotoxins (adjusted OR 1.87; 95% CI 1.21-2.89, p = 0.005). The association between de novo DA and AKI was stronger for severe AKI, i.e. Acute Kidney Injury Network (AKIN) stage 3 (adjusted OR 2.99; 95% CI 1.52-5.85, p = 0.001) and AKIN stage 2 (adjusted OR 1.79; 95% CI 1.002-3.21, p = 0.049) but not AKIN stage 1 (adjusted OR 1.41; 95% CI 0.87-2.29, p = 0.16). CONCLUSIONS: De novo DA within the first 24 h of admission was independently associated with severe AKI in critically ill septic patients. Future studies are required to fully elucidate the utility of DA testing in the early detection and stratification of AKI.