ELKS1 controls mast cell degranulation by regulating the transcription of Stxbp2 and Syntaxin 4 via Kdm2b stabilization.

ELKS1 is a protein with proposed roles in regulated exocytosis in neurons and nuclear factor κB (NF-κB) signaling in cancer cells. However, how these two potential roles come together under physiological settings remain unknown. Since both regulated exocytosis and NF-κB signaling are determinants of mast cell (MC) functions, we generated mice lacking ELKS1 in connective tissue MCs (Elks1f/f Mcpt5-Cre) and found that while ELKS1 is dispensable for NF-κB-mediated cytokine production, it is essential for MC degranulation both in vivo and in vitro. Impaired degranulation was caused by reduced transcription of Syntaxin 4 (STX4) and Syntaxin binding protein 2 (Stxpb2), resulting from a lack of ELKS1-mediated stabilization of lysine-specific demethylase 2B (Kdm2b), which is an essential regulator of STX4 and Stxbp2 transcription. These results suggest a transcriptional role for active-zone proteins like ELKS1 and suggest that they may regulate exocytosis through a novel mechanism involving transcription of key exocytosis proteins.

Molecular determinants of plaque size as an indicator of dengue virus attenuation.

The development of live viral vaccines relies on empirically derived phenotypic criteria, especially small plaque sizes, to indicate attenuation. However, while some candidate vaccines successfully translated into licensed applications, others have failed safety trials, placing vaccine development on a hit-or-miss trajectory. We examined the determinants of small plaque phenotype in two dengue virus (DENV) vaccine candidates, DENV-3 PGMK30FRhL3, which produced acute febrile illness in vaccine recipients, and DENV-2 PDK53, which has a good clinical safety profile. The reasons behind the failure of PGMK30FRhL3 during phase 1 clinical trial, despite meeting the empirically derived criteria of attenuation, have never been systematically investigated. Using in vitro, in vivo and functional genomics approaches, we examined infections by the vaccine and wild-type DENVs, in order to ascertain the different determinants of plaque size. We show that PGMK30FRhL3 produces small plaques on BHK-21 cells due to its slow in vitro growth rate. In contrast, PDK53 replicates rapidly, but is unable to evade antiviral responses that constrain its spread hence also giving rise to small plaques. Therefore, at least two different molecular mechanisms govern the plaque phenotype; determining which mechanism operates to constrain plaque size may be more informative on the safety of live-attenuated vaccines.