Chromogranin B plays an essential role in post-endoplasmic reticulum sorting of dense-core vesicle cargo

Abstract

Chromogranin B (ChgB) is a major dense-core vesicle (DCV) matrix protein whose mechanistic role in DCV function remains ambiguous. To address this, we used CRISPR-Cas9-mediated genetic ablation, combined with quantitative microscopy and biochemical analyses, in PC12 and INS-1 cell lines and mouse models, demonstrating its essential role in DCV function. ChgB knockout cells exhibit severely impaired stimulus-coupled DCV exocytosis, while constitutive and synaptic-like vesicle secretion remain intact. Ultrastructural analyses reveal reduced dense-core area, altered vesicle numbers and sizes, and mislocalisation of DCVs away from the plasma membrane. ChgB deletion further causes Golgi fragmentation and prevents DCV cargo from entering the trans-Golgi network, delaying cargo exit. Subsequently, this possible effect, attributable to augmented endoplasmic reticulum stress, alters the SNARE signature and impairs full-fusion exocytosis. Finally, rescue experiments reversed defective sorting kinetics and exocytosis, suggesting a specific role of ChgB in DCV function. Together, these findings identify ChgB as a central regulator of DCV biogenesis, cargo sorting, and fusion competence.