SummaryBrain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, plays critical roles in the survival, growth, and maintenance of brain and peripheral neurons. We report the presence of BDNF protein in human platelets and its release upon agonist stimulation. The BDNF content of washed platelets varied widely, from 3.5 to 67 ng/ 4 X 108 platelets, averaging 25.2 ± 21.2 ng/4 X 108 platelets (mean ± SD). The BDNF concentration in platelet-poor plasma was low (1.7 ± 1.7 ng/ml, n = 11). Thrombin, collagen, the Ca++ ionophore A23187, and shear stress each induced a rapid release of BDNF from platelets. Up to only half of platelet BDNF was secreted upon agonist stimulation, suggesting that platelets may have a non-releasable pool of BDNF, or that the released BDNF binds to a recognition site on the platelet surface and is internalized, as occurs with serotonin. However, the cognate BDNF receptor, TrkB, was not detected in platelets. Nevertheless, the ability of BDNF to bind washed platelets was shown by FACS analysis confocal microscopy and by the binding and apparent internalization of [125I]-BDNF by platelets. A very high affinity site (Kd = 130 X 10−15 M, ∼80 sites/platelet) and a moderately high affinity site (Kd = 20 nM, ∼3750 sites/platelet) were identified. The BDNF content in two mega-karyocytic cell lines, DAMI and Meg-01, was only 0.1% of the content measured in platelets. No BDNF mRNA was detected by Northern blotting in these cell lines or in platelets. The pituitary gland was also ruled out as a source for platelet BDNF, since the BDNF content of rat platelets did not decrease 2 weeks after hypophysectomy. Thus, platelet BDNF is not acquired from the megakaryocyte or pituitary gland, but is probably acquired from other sources via the blood circulation. Platelets appear to bind, store and release BDNF upon activation at the site of traumatic injury to facilitate the repair of peripheral nerves or other tissues that contain TrkB.

Brain-derived neurotrophic factor (BDNF) is an abundant neurotrophin in brain and peripheral nerves, where it affects neural development, survival and repair after injury. BDNF has been detected in rat and human blood, but the source of circulating BDNF is not established. BDNF messenger and peptide were detected in cultured cells and in the culture medium of human umbilical vein endothelial cells. The expression of BDNF was up-regulated by elevation of intracellular cAMP and down-regulated by Ca 2+ ionophore, bovine brain extract and laminar fluid shear stress. These results suggest that vascular endothelial cells may contribute to circulating BDNF.z 2000 Federation of European Biochemical Societies.

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patient sample is diluted in FV-deficient plasma, appears to provide this increased sensitivity and specificity (2, 3). This modified assay was originally devised for testing o f patients receiving anticoagulant therapy, but it has recently become obvious that (he modified test is highly specific and sensitive for the presence o f the F V : Q 5"1 ' allele. Indeed, in our laboratory we have so far found a perfect correlation between the results o f l ;V genotyping and the modified A l'C-resistanee test (unpublished observation). As a consequence, our current strategy is to analyse all samples with both the original APC-resistancc lest and with the modified APC-resislanee test. Those individuals who present a pathological result in the modified lest are then subjected to l ;V genotyping. This strategy provides us with the required accuracy and security to identify all individuals with the I'V : Q 511' ' allele and at the same time allows us to identify those individuals who have the APC-resislanee phenotype but not the l;V : Q 5"'1 allele.

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