Adhesion to different collagens modulates megakaryocyte development

Megakaryopoiesis occurs in a complex microenvironment within
the bone marrow. The first events occur in the osteoblastic niche
and include commitment of the hemopoietic progenitor cell to
megakaryopoiesis. The second step is megakaryocyte (MK) maturation
and is associated with rapid cytoplasm expansion and
intense synthesis of proteins. Finally MKs, which migrate to the
vascular niche, convert the bulk of their cytoplasm into multiple
long processes called proplatelets that protrude through the vascular
endothelium into the sinusoid lumen, where the platelets
are released. Growing evidence indicates that a complex regulatory
mechanism, involving MK-matrix interactions, may contribute
to the quiescent or permissive microenvironment related to
platelet release within bone marrow. To address this hypothesis,
in this work we have investigated the role of type I, IV and VI
collagens in regulating MK function. Fibrillar type I collagen is
the most abundant extracellular protein of the osteoblastic niche,
while microfibrillar type IV and VI collagens are primary subendothelial
extracellular matrix components. Human MKS (hMKs)
were differentiated from cord blood derived CD34+ cells for
12 days. Mature hMKs were plated onto glass coverslips coated
with type I, IV or VI collagen. Proplatelet formation (PPF) was
evaluated by phase contrast and fluorescence microscopy upon
cell staining with anti-tubulin and CD41 antibodies. Type I, but
not type IV or type VI collagen suppressed PPF. This process
was triggered by the engagement of integrin alpha2beta1 through
activation of Rho/ROCK pathway and myosin-IIA. Moreover,
after a short incubation hMKs were spread on all collagens,
while, prolonging incubation, hMKs on type IV and VI collagens
returned round and started to extend proplatelets, while MKs on
type I collagen remained spread and did not proceed on maturation.
Overall our data represent the first evidence that hMK function
on different collagens may depend on peculiar structural
properties of the collagens, as well as on differences in receptor
engagement.