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Regulation of thrombopoietin levels by c-mpl-mediated binding to platelets
P J Fielder1, A L Gurney, E Stefanich
1Departments of Molecular Oncology, Genentech Inc, South San Francisco, CA USA.
Abstract:
The involvement of platelets and the c-mpl receptor in the regulation of thrombopoietin (TPO) plasma concentrations and tissue mRNA levels was investigated in both normal mice and mice defective in c-mpl (c-mpl-/-). Although c-mpl-/- mice have fewer platelets and higher plasma TPO activity than normal mice, there was no increase in TPO mRNA levels as measured by an S1 nuclease protection assay. After the intravenous injection of 125I-TPO, specific uptake of radioactivity by the spleen and blood cells was present in the normal mice, but absent in the c-mpl-/- mice. Platelet-rich plasma (PRP) from normal mice was able to bind and internalize 125I-TPO, whereas PRP from c-mpl-/- mice lacked this ability. Analysis of 125I-TPO binding to normal PRP indicated that binding was specific and saturable, with an approximate affinity of 560 pmol/L and 220 receptors per platelet. PRP from normal mice was also able to degrade 125I-TPO into lower molecular weight fragments. After the intravenous injections, c-mpl-/- mice cleared a dose of 125I-TPO at a much slower rate than did normal mice. Injection of washed platelets from normal mice into c-mpl-/- mice resulted in a dramatic, but transient, decrease in plasma TPO levels. These data provide evidence that platelets regulate plasma TPO levels via binding to the c-mpl receptor on circulating platelets.
Insights
Platelets regulate thrombopoietin (TPO) levels by binding to the c-mpl receptor. Mice lacking c-mpl receptors show altered TPO levels and impaired TPO clearance, highlighting platelet-specific TPO regulation.
Area of Science:
- Hematology
- Molecular Biology
- Receptor Signaling
Background:
- Thrombopoietin (TPO) is a key regulator of platelet production.
- The c-mpl receptor (also known as TPOR) is expressed on hematopoietic cells, including platelets.
- The precise role of platelets and c-mpl in TPO regulation is not fully understood.
Purpose of the Study:
- To investigate the role of platelets and the c-mpl receptor in regulating thrombopoietin (TPO) plasma concentrations and tissue mRNA levels.
- To determine if c-mpl deficiency affects TPO levels and clearance.
- To elucidate the mechanism by which platelets interact with TPO.
Main Methods:
- Comparison of normal mice and mice deficient in c-mpl (c-mpl-/-).
- Measurement of plasma TPO activity and TPO mRNA levels using S1 nuclease protection assay.
- Intravenous injection of radiolabeled 125I-TPO and assessment of its uptake, binding, internalization, and clearance.
- Analysis of TPO binding kinetics to platelet-rich plasma (PRP).
- Transplantation of normal platelets into c-mpl-/- mice.
Main Results:
- c-mpl-/- mice exhibited fewer platelets and higher plasma TPO activity but no increase in TPO mRNA levels.
- Normal mice showed specific uptake and internalization of 125I-TPO by spleen and blood cells, which was absent in c-mpl-/- mice.
- Platelet-rich plasma (PRP) from normal mice bound and internalized 125I-TPO, while PRP from c-mpl-/- mice did not.
- Normal PRP demonstrated specific, saturable binding of TPO with approximately 220 receptors per platelet and affinity of 560 pmol/L.
- c-mpl-/- mice cleared 125I-TPO significantly slower than normal mice.
- Injection of normal platelets into c-mpl-/- mice caused a transient decrease in plasma TPO levels.
Conclusions:
- Platelets play a crucial role in regulating plasma TPO levels through binding to the c-mpl receptor on circulating platelets.
- The c-mpl receptor on platelets is essential for TPO clearance and homeostasis.
- These findings establish a direct mechanism for platelet-mediated regulation of TPO.