Vesicular protein transport

Four-dimensional multi-particle tracking in living cells based on lifetime imaging

Research on dynamic events in living cells, such as intracellular transportation, is important for understanding cell functions. As movements occur within cells, the microenvironment of the moving vesicles or biomacromolecules may affect the behavior of them. Herein, we propose a method of simultaneously monitoring changes in spatial positions and the local environment related to the fluorescence lifetime, i.e., four-dimensional (4D) multi-particle parallel-tracking in living cells. Based on double-helix point spread function (DH-PSF) microscopy and streak camera, the method combines three-dimensional (3D) localization methods and fluorescence lifetime imaging. By modifying the PSF of the system, the 3D positions and fluorescence lifetime information for several molecules within a depth of a few microns can be acquired simultaneously from a single snapshot. The feasibility of this method is verified by simulating the real-time tracking of a single particle with a given trajectory. In addition, a proof-of-concept 4D tracking system based on the DH-PSF and streak camera was built. The experimental results show that the 3D localization and lifetime precision are σ(x, y, z) = (26 nm, 35 nm, 53 nm) and σ(τ) = 103 ps, respectively, and the effective depth of field is approximately 4 μm. Finally, intracellular endocytosis in a living cell was observed using the system, which demonstrated the successful 4D tracking of two microspheres moving within an axial depth of 4 μm. This work opens a new perspective for research of dynamic processes, by providing information about the chemical (microenvironments) and physical (positions) changes of moving targets in living cells.

Mutations in the coat complex II component SEC23B promote colorectal cancer metastasis

Metastasis is the leading cause of death for colorectal cancer (CRC). However, the protein transport process involved in CRC metastasis remains unclear. In this report, we use whole-exome sequencing and bioinformatics analysis to identify somatic mutations in CRC samples and found mutations of the protein transport gene Sec23 homolog B (SEC23B) in patients with metachronous liver metastasis. We show that deletion of SEC23B suppresses the membrane localization of adhesion proteins and augments cell mobility. SEC23B mutations either cause a premature stop (C649T) or impair its protein transport activity (C1467G and T488C + G791A + G2153A). Furthermore, SEC23B mutations inhibit the transport of epithelial cell adhesion molecule (EPCAM) and CD9 molecule, thereby attenuating cell adhesion and promoting invasiveness both in vitro and in vivo. Taken together, these data demonstrate the important impact of SEC23B mutations on metastasis, and we propose that SEC23B is a potential suppressor of CRC metastasis.

The cytosolic adaptor AP-1A is essential for the trafficking and function of Niemann-Pick type C proteins

Niemann-Pick type C (NPC) disease is a fatal neurodegenerative disorder characterized by over-accumulation of low-density lipoprotein-derived cholesterol and glycosphingolipids in late endosomes/lysosomes (LE/L) throughout the body. Human mutations in either NPC1 or NPC2 genes have been directly associated with impaired cholesterol efflux from LE/L. Independent from its role in cholesterol homeostasis and its NPC2 partner, NPC1 was unexpectedly identified as a critical player controlling intracellular entry of filoviruses such as Ebola. In this study, a yeast three-hybrid system revealed that the NPC1 cytoplasmic tail directly interacts with the clathrin adaptor protein AP-1 via its acidic/di-leucine motif. Consequently, a nonfunctional AP-1A cytosolic complex resulted in a typical NPC-like phenotype mainly due to a direct impairment of NPC1 trafficking to LE/L and a partial secretion of NPC2. Furthermore, the mislocalization of NPC1 was not due to cholesterol accumulation in LE/L, as it was not rescued upon treatment with Mβ-cyclodextrin, which almost completely eliminated intracellular free cholesterol. Our cumulative data demonstrate that the cytosolic clathrin adaptor AP-1A is essential for the lysosomal targeting and function of NPC1 and NPC2.