NPR-B Expression In Megakaryocytes and Platelets

Abstract 4319

C-type natriuretic peptide (CNP) is known to exert pleiotropic effects on a variety of cell types through binding to its receptor, natriuretic peptide receptor type B (NPR-B) and activation of the cytoplasmic intracellular guanylyl cyclase domain. Although nitric oxide-mediated activation of cyclic guanosine monophosphate (cGMP) is known to induce megakaryocyte apoptosis and regulate platelet activation, the effects of CNP on these cells is yet to be defined. In this study for the first time we have addressed the potential role of CNP and NPR-B in megakaryocyte and platelet biology.

We identified NPR-B on murine megakaryocytes by immunostaining demonstrating co-localization of CD41 and NPR-B. CNP induced cGMP production in megakaryocytes compared to a mutant form of CNP containing a single amino acid substitution (Lbab) (67.24 ± 10.65 fmol/well vs. 17.18 ± 2.089 fmol/well; P<0.05). Subsequently, CNP was shown to reduce megakaryocyte colony formation compared to Lbab and this effect was dose-dependent: Lbab 500nM 26.75 ± 5.618 CFU-M vs. 15.25 ± 2.750 CFU-M for CNP 500nM and Lbab 1000 nM 27.50 ± 2.102 CFU-M vs. 12.25 ± 1.493 CFU-M for CNP 1000nM; P<0.05). Furthermore, CNP also resulted in smaller-sized colonies, with altered cell morphology.

To verify these findings, wild-type C57/BL6 mice were treated with daily injections of either saline, 1uM CNP or 1uM Lbab for one week, following which megakaryocytes were isolated and cultured. Colony formation was diminished in mice injected with CNP compared to mice receiving saline or Lbab peptide (125.3 ± 18.54 CFU-M for saline; 107.0 CFU-M ± 20.04 for Lbab and 20.00 ± 3.674 CFU-M for CNP; P<0.05). In addition platelet numbers were measured in the aforementioned mice and the CNP treated mice had significantly lower platelet counts compared to the mice treated with Lbab (832.0 ± 13.19 ×10³/ul for CNP vs. 778.0 ± 12.00 ×10³/ul for Lbab; P<0.05). Taken together, these data indicate that CNP may play a role in the development, maturation and/or survival of megakaryocytes.

Using flow cytometry we identified that 47% of CD41⁺ murine platelets co-express NPR-B. In addition, this expression of NPR-B was confirmed by immunocytostaining and western blotting in both human and murine platelets. Interestingly, despite the expression of NPR-B, and unlike megakaryocytes, CNP (similar to the mutant peptide) did not increase cGMP production in platelets (1.652 ± 0.016 fmol/well for CNP and 1.606 ± 0.01550 fmol/well for Lbab). Further, although preliminary results did not indicate that CNP alters platelet secretion or aggregation, exposure to the peptide profoundly inhibited adhesion of platelets to collagen (53 ± 5 adhered platelets with Lbab, 32 ± 3 with CNP, relative reduction 42%; P<0.05).

In summary, these results provide for the first time evidence that the NPR-B receptor is expressed on both megakaryocytes and platelets. Our studies suggest that CNP might have both cGMP dependent and independent functions in megakaryocytes and platelets and further studies will assist in delineating the mechanisms by which CNP exerts its actions on these cells.