Intrinsic impaired proplatelet formation and microtubule coil assembly of megakaryocytes in a mouse model of Bernard-Soulier syndrome.
BACKGROUND
Giant platelets and thrombocytopenia are invariable defects in the Bernard-Soulier syndrome caused by deficiency of the GPIb-V-IX complex, a receptor for von Willebrand factor supporting platelet adhesion to the damaged arterial wall. Various properties of this receptor may be considered potential determinants of the macrothrombocytopenia.


METHODS
To explore the underlying mechanisms of the disease, megakaryopoiesis was studied in a mouse model deficient in GPIbbeta. Megakaryocytes were initially characterized in situ in the bone marrow of adult mice, after which their capacity to differentiate into proplatelet-bearing cells was evaluated in cultured fetal liver cells.


RESULTS
The number of megakaryocyte progenitors, their differentiation and progressive maturation into distinct classes and their level of endoreplication were normal in GPIbbeta(-/-) bone marrow. However, the more mature cells exhibited ultrastructural anomalies with a thicker peripheral zone and a less well developed demarcation membrane system. GPIbbeta(-/-) megakaryocytes could be differentiated in culture from Lin(-) fetal liver cells in normal amounts but the proportion of cells able to extend proplatelets was decreased by 41%. Moreover, the GPIbbeta(-/-) cells extending proplatelets displayed an abnormal morphology characterized by fewer pseudopodial extensions with thicker shaft sections and an increased diameter of the terminal coiled elements. GPIbbeta(-/-) released platelets were larger but retained a typical discoid shape. Proplatelet formation was similarly affected in bone marrow explants from adult mice examined by videomicroscopy. The marginal microtubular ring contained twice as many tubulin fibers in GPIbbeta(-/-) proplatelet buds in cultured and circulating platelets.


CONCLUSIONS
Altogether, these findings point to a role of the GPIb-V-IX complex intrinsic to megakaryocytes at the stage of proplatelet formation and suggest a functional link with the underlying microtubular cytoskeleton in platelet biogenesis.
INSERM U949, EFS-Alsace, Strasbourg.
Catherine
C
Strassel
Anita
A
Eckly
Catherine
C
Léon
Claire
C
Petitjean
Monique
M
Freund
Jean-Pierre
JP
Cazenave
Christian
C
Gachet
François
F
Lanza
D000328:Adult; D000818:Animals; D001606:Bernard-Soulier Syndrome / Q000378:metabolism* / Q000473:pathology; D001792:Blood Platelets / Q000166:cytology / Q000378:metabolism; D001854:Bone Marrow Cells / Q000166:cytology / Q000378:metabolism; D002470:Cell Survival; D004195:Disease Models, Animal; D005260:Female; D005434:Flow Cytometry; D006801:Humans; D007700:Kinetics; D055016:Megakaryocyte Progenitor Cells / Q000166:cytology / Q000378:metabolism*; D008533:Megakaryocytes / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D051379:Mice; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D008856:Microscopy, Fluorescence; D008870:Microtubules / Q000378:metabolism*; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000235:genetics / Q000378:metabolism; D013997:Time Factors; D055815:Young Adult
D019038:Platelet Glycoprotein GPIb-IX Complex