Studies of mice with genetic deficiencies have provided important insights on the functions of many proteins in thrombosis and hemostasis. Recently, a strain of mice (C57BL/6JOlaHsd, an inbred strain of C57BL/6J) has been identified to have a spontaneous, tandem deletion of the multimerin 1 and α-synuclein genes, which are also adjacent genes on human chromosome 4q22. Multimerin 1 is an adhesive protein found in platelets and endothelial cells while α-synuclein is a protein found in the brain and in blood that is implicated in neurodegenerative diseases and exocytosis. In vitro, multimerin 1 supports platelet adhesion while α-synuclein inhibits α-granule release. We postulated that the loss of multimerin 1 and α-synuclein would alter platelet function and that recombinant human multimerin 1 might correct some of these abnormalities. We compared platelet adhesion, aggregation and thrombus formation in vitro and in vivo in C57BL/6JOlaHsd and C57BL/6 mice. Thrombus formation was studied by using the ferric-chloride injured mesenteric arteriole thrombosis model under intravital microscopy. We found that platelet adhesion, aggregation and thrombus formation in C57BL/6JOlaHsd were significantly impaired in comparison to control, C57BL/6 mice. The number of single platelets, deposited 3–5 minutes after injury, was significantly decreased in C57BL/6JOlaHsd mice (P <0.05, platelets/min: C57BL/6 = 157 ± 15, n=16; C57BL/6JOlaHsd = 77 ± 13, n=17). Moreover, thrombus formation in these mice was significantly delayed. Thrombi in C57BL/6JOlaHsd were unstable and easily dissolved, which resulted in significant delays (P<0.001) in vessel occlusion (mean occlusion times: C57BL/6 = 15.6 ± 1.2 min, n=16; C57BL/6JOlaHsd = 31.9 ± 2.1 min, n=17). We further tested platelet function in these mice by ADP and thrombin induced platelet aggregation using platelet rich plasma and gel-filtered platelets, respectively. Although no significant differences were seen with ADP aggregation, thrombin-induced platelet aggregation was significantly impaired in C57BL/6JOlaHsd mice. Platelet adhesion to type I collagen (evaluated using microcapillary chambers, perfused at 1500 s−1 with whole blood) was also impaired in C57BL/6JOlaHsd mice. However, platelets from C57BL/6JOlaHsd mice showed a normal pattern of agonist-induced release of α-granule P-selectin. Multimerin 1 corrected the in vitro aggregation and adhesion defects of C57BL/6JOlaHsd platelets. Furthermore, the transfusion of multimerin 1 into C57BL/6JOlaHsd mice corrected the impaired platelet deposition and thrombus formation in vivo. No significant difference was found in tail bleeding time between the two groups of mice. As α-synuclein knockout mice have a shortened time to thrombus formation (

), the effects of multimerin 1 on impaired platelet function in C57BL/6JOlaHsd mice provide supportive evidence that multimerin 1 contributes to platelet adhesion and thrombus formation at the site of vessel injury. The findings suggest multimerin 1 knockout mice will be useful to explore platelet function. The first two authors and participating laboratories contributed equally to this study.

Disclosures: No relevant conflicts of interest to declare.

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