The
tyrosine kinase JAK2 is a critical component of intracellular JAK/STAT
cytokine signaling cascades that is prevalent in hematopoietic cells, such as hematopoietic stem cells and megakaryocytes (MKs). Individuals expressing the somatic JAK2 V617F mutation commonly develop myeloproliferative
neoplasms (MPNs) associated with venous and arterial
thrombosis, a leading cause of mortality. The role of JAK2 in hemostasis remains unclear. We investigated the role of JAK2 in platelet
hemostatic function using Jak2fl/fl Pf4-Cre (Jak2Plt-/-) mice lacking JAK2 in platelets and MKs. Jak2Plt-/- mice developed MK
hyperplasia and
splenomegaly associated with severe
thrombocytosis and
bleeding. This notion was supported by failure to occlude in a
ferric chloride carotid artery injury model and by a cremaster muscle
laser-induced injury assay, in which Jak2Plt-/- platelets failed to form stable thrombi. Jak2Plt-/- platelets formed thrombi poorly after adhesion to
type 1 collagen under arterial shear rates. Jak2Plt-/- platelets spread poorly on
collagen under static conditions or on
fibrinogen in response to the
collagen receptor GPVI-specific agonist,
collagen-related peptide (CRP). After activation with
collagen, CRP, or the CLEC-2 agonist rhodocytin, Jak2Plt-/- platelets displayed decreased α-granule secretion and
integrin αIIbβ3 activation or aggregation, but showed normal responses to
thrombin. Jak2Plt-/- platelets had impaired intracellular signaling when activated via GPVI, as assessed by
tyrosine phosphorylation. Together, the results show that JAK2 deletion impairs platelet immunoreceptor tyrosine-based activation motif signaling and
hemostatic function in mice and suggest that aberrant JAK2 signaling in patients with MPNs affects GPVI signaling, leading to
hemostatic platelet function.