Abstract
Congenital amegakaryocytic thrombocytopaenia (CAMT) is a disorder caused by c-MPL
mutations that impair thrombopoietin (TPO) signalling, resulting in a near absence
of megakaryocytes (MKs). While this phenotype is consistent in adults, neonates with
CAMT can present with severe thrombocytopaenia despite normal MK numbers. To investigate
this, we characterized MKs and platelets in newborn c-MPL
–/– mice. Liver MKs in c-MPL
–/– neonates were reduced in number and size compared with wild-type (WT) age-matched
MKs, and exhibited ultrastructural abnormalities not found in adult c-MPL
–/– MKs. Platelet counts were lower in c-MPL
–/– compared with WT mice at birth and did not increase over the first 2 weeks of life.
In vivo biotinylation revealed a significant reduction in the platelet half-life of
c-MPL
–/– newborn mice (P2) compared with age-matched WT pups, which was not associated with
ultrastructural abnormalities. Genetic deletion of the pro-apoptotic Bak did not rescue
the severely reduced platelet half-life of c-MPL
–/– newborn mice, suggesting that it was due to factors other than platelets entering
apoptosis early. Indeed, adult GFP+ (green fluorescent protein transgenic) platelets
transfused into thrombocytopenic c-MPL
–/– P2 pups also had a shortened lifespan, indicating the importance of cell-extrinsic
factors. In addition, neonatal platelets from WT and c-MPL
–/– mice exhibited reduced P-selectin surface expression following stimulation compared
with adult platelets of either genotype, and platelets from c-MPL
–/– neonates exhibited reduced glycoprotein IIb/IIIa (GPIIb/IIIa) activation in response
to thrombin compared with age-matched WT platelets. Taken together, our findings indicate
that c-MPL deficiency is associated with abnormal maturation of neonatal MKs and developmental
stage-specific defects in platelet function.
Keywords
animal models - bone marrow - megakaryocytes - platelet physiology - thrombocytopaenia
