α-granule protein deficiencies in ARC megakaryocytes and platelets. Immunoblots comparing megakaryocyte (MK) and platelet (PLT) whole-cell lysates or plasma from ARC and normal (N) fetal, neonatal, and adult sources as indicated for each lane. (A) Markedly reduced PF4 content in ARC fetal megakaryocytes (lane 1) compared with control fetal megakaryocytes (lanes 2, 3), control adult platelets (lane 4), and megakaryocytes (lane 5) were observed (reduced 10% SDS-PAGE). Lysate from equivalent numbers of megakaryocytes (CD41⁺-labeled cells) was loaded for lanes 1 to 3. Equivalent protein loading is indicated by visualizing actin on the same blot. (B) Significantly decreased PF4 was also observed in ARC neonatal platelets (lane 1) compared with normal neonatal (lanes 2, 3) and adult (lane 4) platelets (reduced 15% SDS-PAGE; PF4 and actin were probed and developed simultaneously). (C) Platelet VWF is undetectable in ARC platelets (lane 1) compared with normal neonatal (lanes 2, 3) and adult (lane 4) platelets (reduced 8% SDS-PAGE). (D) In contrast, VWF was present at roughly equivalent quantities (0.5 μL plasma/lane) in plasma from patients with ARC syndrome (lane 1) compared with normal neonatal (lanes 2, 3) and adult (lane 4) control plasma (reduced 8% SDS-PAGE).