title: GPIbα regulates platelet size by controlling the subcellular localization of filamin.
abstract: Interaction between the cytoplasmic domain of GPIbα with its cytoskeletal binding partner, filamin, is a major determinant of platelet size, and deficiency of either protein results in macrothrombocytopenia. To clarify the mechanism by which GPIbα-filamin interactions regulate platelet production, we manipulated the expression levels of filamin and GPIb in cultured embryonic stem cells (ESCs) that were subsequently differentiated into platelets. Knocking down filamins A and B resulted in the production of ESC-derived proplatelets with abnormally large swellings and proplatelet shafts that generated giant platelets in culture. Large platelets could also be generated by overexpressing GPIbα in ESCs, or by overexpressing in vivo a transgene encoding a chimeric protein containing the cytoplasmic domain of GPIbα. To identify the mechanism by which the GPIb:filamin ratio regulates platelet size, we manipulated filamin and GPIbα levels in HEK293T cells and examined the effects of overexpressing either protein on their ability to traffic to the cell periphery. Accumulation of either protein within the endoplasmic reticulum resulted in trapping of the other. Taken together, these data demonstrate that coordinated expression of GPIbα and filamin is required for efficient trafficking of either protein to the cell surface, and for production of normal-sized platelets.
authors.affiliation: Blood Research Institute, Blood Center of Wisconsin, Milwaukee, WI 53266-3548, USA. Taisuke.Kanaji@bcw.edu
authors.forename: Taisuke
authors.initials: T
authors.lastname: Kanaji
authors.forename: Jerry
authors.initials: J
authors.lastname: Ware
authors.forename: Takashi
authors.initials: T
authors.lastname: Okamura
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Newman
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000166:cytology* / Q000378:metabolism*; D002454:Cell Differentiation / Q000502:physiology*; D003285:Contractile Proteins / Q000378:metabolism*; D064448:Filamins; D005434:Flow Cytometry; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D008822:Mice, Transgenic; D008840:Microfilament Proteins / Q000378:metabolism*; D019038:Platelet Glycoprotein GPIb-IX Complex; D021381:Protein Transport / Q000502:physiology; D034061:Thrombopoiesis / Q000502:physiology*
chemical_list: D003285:Contractile Proteins; D064448:Filamins; D008562:Membrane Glycoproteins; D008840:Microfilament Proteins; D019038:Platelet Glycoprotein GPIb-IX Complex; C094990:adhesion receptor