Targeting cargo to an unconventional secretory system within megakaryocytes allows the release of transgenic proteins from platelets

Background Platelets are essential for hemostasis, and thrombosis, and play vital roles during metastatic cancer progression and infection. Hallmarks of platelet function are activation, cytoskeletal rearrangements, and the degranulation of their cellular contents upon stimulation. While alpha and dense granules are the most studied platelet secretory granules, the dense tubular system (DTS) also functions as a secretory system for vascular thiol isomerases. However, how DTS cargo is packaged and transported from megakaryocytes (MKs) to platelets is poorly understood. Objectives To underpin the mechanisms responsible for DTS cargo transport and leverage those for therapeutic protein packaging into platelets. Methods A retroviral expression system combined with immunofluorescence confocal microscopy was employed to track protein DTS cargo protein disulfide isomerase (PDI) fused to eGFP (eGFP-PDI) during platelet production. Murine bone marrow transplantation models were used to determine the release of therapeutic proteins from platelets. Results and conclusions We demonstrated that the endoplasmic reticulum retrieval motif Lys-Asp-Glu-Leu (KDEL) located at the C-terminus of PDI was essential for the regular transport of eGFP-PDI-containing granules. eGFP-PDIΔKDEL, in which the retrieval signal was deleted, was aberrantly packaged and its expression was upregulated within clathrin-coated endosomes. Finally, we found that ectopic transgenic proteins, such as tissue factor pathway inhibitor and interleukin 2, can be packaged into MKs and proplatelets by adding a KDEL retrieval sequence. Our data corroborate the DTS as a non-canonical secretory system in platelets and demonstrate that in vitro-generated MKs and platelets may be used as a delivery system for transgenic proteins during cellular therapy.