Fig. 4

Onset of Blood Circulation Is Dependent on Metalloproteases and on the Flow

(A and B) Mouse ADAM8 (mADAM8), mADAM8 EQ, zADAM8, zADAM8 EQ, or empty vector cDNA was coexpressed with mPSGL-1-HA (A) or zPSGL-1-HA (B) in HEK293T cells. Ectodomain shedding was assessed by western blotting for HA. Signals for long forms (L-L, dimer; L, monomer) were decreased and those for short forms (S) were increased significantly by expression of mADAM8 and zADAM8, but not by expression of EQ mutants. Dimeric forms cannot be found with zPSGL-1, probably because the N-terminal region missing in zPSGL-1 is required for the dimerization of mPSGL-1. Asterisks indicate intracellular forms.

(C) A working hypothesis on the onset of primitive blood circulation. As a first step, erythroid cells adhere to the lumen of the vasculature after intravasation until the majority of blood cells transmigrate into the vasculature (“idling”). Next, these cells are released almost simultaneously through two conceivable pathways. In the first, ADAM8 activation in response to a threshold flow velocity or by some stimulus emanating from the vessel lumen causes detachment of erythroid cells and triggers the synchronous onset of blood circulation. In the second, ADAM8-dependent abrogation of erythroid cell adhesion (blue lines) to the blood vessels is a prerequisite, but not a direct trigger, for the synchronous onset of blood circulation. In this case, other stimuli would still be needed for release, such as a threshold flow or the removal of unknown molecules on the vasculature that prevent the release of blood cells. When the heartbeat starts, regurgitation of the flow occurs as a result of the heart valve immaturity. Thus, a unidirectional threshold flow would be generated by maturation of both the heart valve and the heart muscle. The threshold flow would then facilitate flattening and smoothing of blood vessel lumens and/or degradation of extracellular matrix proteins on the vessels that might loosely trap blood cells. Such an abrupt alteration in the vessel could be a direct trigger for the sudden and simultaneous onset of primitive blood circulation.