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Purpose of Review: Platelets are specialized effector cells that rapidly respond to sites of vascular injury. However, emerging data demonstrate that platelets possess diverse functions that also mediate inflammatory responses and neurological diseases. These functions are relevant to disease processes prevalent among older adults and likely influence susceptibility to thrombotic and inflammatory disorders., Recent Findings: Platelet counts decrease in aged individuals whereas platelet reactivity increases. The platelet transcriptome is altered in aged individuals resulting in altered platelet function and exaggerated inflammation. Platelet signaling to monocytes in aging results in significantly more cytokines because of increased platelet-derived granzyme A. Platelet activation in aging appears to be driven, in part, because of increased reactive oxygen species and activation of the mammalian target of rapamycin pathway. Increased platelet hyperactivity in diseases is associated with aging, such cardiovascular disease and sepsis, exaggerate inflammation and thrombosis. Noncanonical functions of platelets influence the development of neurological diseases including Alzheimer's disease., Summary: Although there have been advances dissecting the molecular mechanisms regarding aging-related changes in platelets, many knowledge gaps still remain. Studies filling these gaps are likely to identify new mechanisms driving aging-related changes in platelet gene expression and function, and contributing to injurious thrombo-inflammation in older adults.

Aging and chronic inflammation are independent risk factors for the development of atherothrombosis and cardiovascular disease. We hypothesized that aging-associated inflammation promotes the development of platelet hyperreactivity and increases thrombotic risk during aging. Functional platelet studies in aged-frail adults and old mice demonstrated that their platelets are hyperreactive and form larger thrombi. We identified tumor necrosis factor α (TNF-α) as the key aging-associated proinflammatory cytokine responsible for platelet hyperreactivity. We further showed that platelet hyperreactivity is neutralized by abrogating signaling through TNF-α receptors in vivo in a mouse model of aging. Analysis of the bone marrow compartments showed significant platelet-biased hematopoiesis in old mice reflected by increased megakaryocyte-committed progenitor cells, megakaryocyte ploidy status, and thrombocytosis. Single-cell RNA-sequencing analysis of native mouse megakaryocytes showed significant reprogramming of inflammatory, metabolic, and mitochondrial gene pathways in old mice that appeared to play a significant role in determining platelet hyperreactivity. Platelets from old mice (where TNF-α was endogenously increased) and from young mice exposed to exogenous TNF-α exhibited significant mitochondrial changes characterized by elevated mitochondrial mass and increased oxygen consumption during activation. These mitochondrial changes were mitigated upon TNF-α blockade. Similar increases in platelet mitochondrial mass were seen in platelets from patients with myeloproliferative neoplasms, where TNF-α levels are also increased. Furthermore, metabolomics studies of platelets from young and old mice demonstrated age-dependent metabolic profiles that may differentially poise platelets for activation. Altogether, we present previously unrecognized evidence that TNF-α critically regulates megakaryocytes resident in the bone marrow niche and aging-associated platelet hyperreactivity and thrombosis., (© 2019 by The American Society of Hematology.)