Bioprotective Role of Platelet-Derived Microvesicles in Hypothermia: Insights from the Differential Characteristics of Peripheral and Splenic Platelets

While platelets play an important role in hemostasis and blood coagulation, they also contribute to tissue repair. Most platelets are present in peripheral blood, but some are stored in the spleen. Since tissue environments of peripheral blood vessels and the spleen are quite distinctive, the properties of the platelets present in each may also differ. However, no studies have addressed this difference.We have reported that accidental hypothermia activates splenic platelets, but not peripheral blood, whose biological significance is unknown. Accidental hypothermia is an involuntary drop in core body temperature to less than 35°C due to exposure to low environmental temperatures. Shivering, an involuntary small contraction of skeletal muscles throughout the body, is thought to be an automatic defense system against cold, in which skeletal muscles contract to increase heat production. Shivering occurs in muscles throughout the body, so when muscles are damaged by it, the tissue must be repaired by a mechanism that covers the entire body.Platelets, when activated, release microvesicles that encapsulate many bioactive substances. These microvesicles are called platelet-derived microvesicles (PDMV). It is generally believed that when cells take up extracellular vesicles, they are affected by the action of the encapsulated substances. Therefore, we hypothesized that PDMV released by platelet activation in the spleen at hypothermia may be involved in the regeneration of skeletal muscle throughout the body. C57Bl/6 mice were placed in an environment of -20°C and the rectal temperature was lowered to 15°C to model hypothermia. Platelets, PDMV, and skeletal muscle tissue were collected from these mice for analysis.Transcriptomic changes between splenic and peripheral platelets were greater in hypothermic mice than in normal mice. Pathway analysis of these Transcriptomic changes showed alterations related to Wnt signaling, cytoskeletal remodeling, and cell adhesion. Electron microscopy and real-time RT-PCR analysis revealed that platelets activated in the spleen by hypothermia internalize the transcripts involved in tissue repair, such as Wnt3a and Ctnnb1, into PDMV and release them into plasma. Plasma microvesicles from hypothermic mice promoted wound healing in the mouse myoblast cell line C2C12. Skeletal muscles of hypothermic mice were damaged but recovered within 24 hours after rewarming. However, splenectomy delayed recovery from skeletal muscle injury even after the mice were rewarmed.These results indicate that PDMV released from activated platelets in the spleen plays an important role in the repair of skeletal muscle damaged by shivering of hypothermia, suggesting a novel function of the spleen not previously known.