Gut Microbiome and Circadian Interactions with Platelets Across Human Diseases, including Alzheimer's Disease,...

Platelets have traditionally been investigated for their role in clot formation in the course of cardiovascular diseases and strokes. However, recent work indicates platelets to be an integral aspect of wider systemic processes, with relevance to the pathophysiology of a host of diverse medical conditions, including neurodegenerative disorders and cancer. This article reviews platelet function and interactions with the gut microbiome and circadian systems, highlighting the role of the platelet mitochondrial melatonergic pathway in determining platelet activation, fluxes and plasticity. This provides a number of novel conceptualizations of platelet function and mode of interaction with other cell types, including in the pathoetiology and pathophysiology of diverse medical conditions, such as cancer, Alzheimer's disease, and amyotrophic lateral sclerosis. It is proposed that a platelet-gut axis allows platelets to contribute to many of the pathophysiological processes linked to gut dysbiosis and gut permeability. This is at least partly via platelet sphingosine- 1-phosphate release, which regulates enteric glial cells and lymphocyte chemotaxis, indicating an etiological role for platelets in a wide array of medical conditions linked to alterations in the gut microbiome. Platelets are also an important regulator of the various microenvironments that underpin most human medical conditions, including the tumor microenvironment, neurodegenerative diseases, and autoimmune disorders. Platelet serotonin release regulates the availability of the mitochondrial melatonergic pathway systemically, thereby being an important determinant of the dynamic metabolic interactions occurring across cell types that underpin the pathoetiology of many medical conditions. In addition, a number of novel and diverse future research directions and treatment implications are proposed.