VPS33B regulates protein sorting into and maturation of α-granule progenitor organelles in mouse megakaryocytes

Abstract

Key Points• Hematologic effects in the mouse model for ARC syndrome, Vps33b fl/fl -ER T2, in which Vps33b is ubiquitously excised post-development.• The VPS33B-VIPAR complex is responsible for sorting cargo to and maturation of a-granule-destined MVBs.Arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome is caused by deficiencies in the trafficking proteins VPS33B or VIPAR, and is associated with a bleeding diathesis and a marked reduction in platelet a-granules. We generated a tamoxifen-inducible mouse model of VPS33B deficiency, Vps33b

Characterization of the Mammalian CORVET and HOPS Complexes and Their Modular Restructuring for Endosome Specificity

“…Under steady-state conditions VPS33B and VIPAR do not significantly localize to LE (22), but we have shown that these proteins can be recruited to LE by RILP (30). In line with a function of VPS33B and VIPAR at the LE, it was recently shown that VPS33B is important for the maturation of the ␣-granule, a specialized late endosomal compartment (34). Therefore, we investigated whether pathogenic mutations in VPS33B could still be recruited to LE by RILP.…”

Should any genetic defect affecting α‐granules in platelets be classified as gray platelet syndrome?

“…It is caused by mutations in VPS33B or VIPAS39; genes that encode two vesicle-mediated protein-sorting proteins (VPS33B and VIPAR) that form a functional complex. Studies on MKs in a conditional mouse model of ARC syndrome have confirmed abnormalities in protein trafficking from the trans Golgi network or via endocytosis and in the maturation of multivesicular bodies, a subset of late endosomes and precursors of agranules [21]. Only a restricted population of small a-granule-like structures is present in platelets from ARC patients and, distinctively, P-selectin is severely decreased.…”

Sorting protein VPS33B regulates exosomal autocrine signaling to mediate hematopoiesis and leukemogenesis

“…To determine potential molecules that play important roles in exosome secretion, we screened several potential candidates that previous studies have suggested may participate in vesicle trafficking, including VPS16B, VPS33B, RAB11A, and RAB27A (22,27,29,33). We found that VPS16B and VPS33B, but not RAB11A or RAB27A (Supplemental Figure 1D), were highly enriched in human plasma-derived exosomes ( Figure 1F).…”

“…We found that VPS16B and VPS33B, but not RAB11A or RAB27A (Supplemental Figure 1D), were highly enriched in human plasma-derived exosomes ( Figure 1F). Because VPS33B has been reported to be a regulator of MVB maturation (22,27,28), we speculated that secretory proteins may be associated with VPS33B. Therefore, we examined the colocalization of VPS33B and several groups of candidate proteins known to be critical for HSC stemness (TPO, ANGPTL2, ANGPTL3, LDHA, and PKM2) (8,14,15,34,35) as well as other potential HSC stemness regulators (GLUT1 and other members of the ANGPTL family) (34,36) using immunofluorescence staining.…”

“…In most cases, contents derived from either receptor-mediated endocytosis or endogenous synthesis in the Golgi apparatus are fused and modified within early endosomes, which can be further processed to maturation in late endosomes/lysosomes or MVBs (a prerequisite for exosome formation). Two types of MVBs have been identified according to their internal morphology: type I MVBs (MVB I), which contain abundant ILVs, and type II MVBs (MVB II), which contain electron-dense material as well as ILVs (22). Studies have revealed that vesicle trafficking is stringently controlled by a number of molecules.…”