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

D Bem, H Smith, B Banushi, JJ Burden… - Blood, The Journal …, 2015 - ashpublications.org
D Bem, H Smith, B Banushi, JJ Burden, IJ White, J Hanley, N Jeremiah, F Rieux-Laucat…
Blood, The Journal of the American Society of Hematology, 2015ashpublications.org
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 α-granules. We generated a tamoxifen-
inducible mouse model of VPS33B deficiency, Vps33b fl/fl-ER T2, and studied the platelet
phenotype and α-granule biogenesis. Ultrastructural analysis of Vps33b fl/fl-ER T2 platelets
identified a marked reduction in α-granule count and the presence of small granule-like …
Abstract
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 α-granules. We generated a tamoxifen-inducible mouse model of VPS33B deficiency, Vps33bfl/fl-ERT2, and studied the platelet phenotype and α-granule biogenesis. Ultrastructural analysis of Vps33bfl/fl-ERT2 platelets identified a marked reduction in α-granule count and the presence of small granule-like structures in agreement with the platelet phenotype observed in ARC patients. A reduction of ∼65% to 75% was observed in the α-granule proteins von Willebrand factor and P-selectin. Although platelet aggregation responses were not affected, a defect in δ-granule secretion was observed. Under arteriolar shear conditions, Vps33bfl/fl-ERT2 platelets were unable to form stable aggregates, and tail-bleeding measurement revealed a bleeding diathesis. Analysis of bone marrow-derived megakaryocytes (MKs) by conventional and immuno-electron microscopy from Vps33bfl/fl-ERT2 mice revealed a reduction in mature type-II multivesicular bodies (MVB II) and an accumulation of large vacuoles. Proteins that are normally stored in α-granules were underrepresented in MVB II and proplatelet extensions. These results demonstrate that abnormal protein trafficking and impairment in MVB maturation in MKs underlie the α-granule deficiency in Vps33bfl/fl-ERT2 mouse and ARC patients.
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