Podocalyxin in rat platelets and megakaryocytes

Podocalyxin is a membrane protein of rat podocytes and endothelial cells, I t has not been described in other cell types, and no amino acid or DNA sequ ence data are available about it, Here we show that podocalyxin antigens ar e present In rat platelets and megakaryocytes, In resting platelets, the an tigens are mainly intracellular but become surface exposed after thrombin s timulation, as shown by immunofluorescence and flow cytometry, By Western b lotting, platelet podocalyxin has an apparent M-r of 140,000, Cytocentrifug e slides of rat bone marrow show that antipodocalyxin antibodies recognize large polyploid cells also expressing CD62P, indicating that the cells are megakaryocytes. From a rat glomerular cDNA library we isolated a clone cove ring the carboxyl-terminal nucleotides of rat podocalyxin. Its putative tra nsmembrane or intracellular domains are 100% or >93% identical, respectivel y, with the human and rabbit podocalyxin-like proteins. The truncated extra cellular domain extends to include two of the four conserved cysteines shar ed by podocalyxin-like proteins. By Northern blotting, a 5.5-kb renal corti cal transcript is seen. By in situ hybridization, cRNA probes recognize pod ocytes, endothelial cells, and megakaryocytes, and by reverse transcription polymerase chain reaction, platelets are shown to contain podocalyxin mRNA . Our results show that rat podocalyxin is a homologue of the previously cl oned podocalyxin-like proteins and suggest that also in mammals podocalyxin has a role in hematopoiesis, as previously shown in the chicken.