Extracellular vesicles and the Stress System

Extracellular vesicles (EVs) are membrane-enclosed nanoparticles that contain various biomolecules, including nucleic acids, proteins, and lipids, and are manufactured and released by virtually all cell types. There is evidence that EVs are involved in intercellular communication, acting in an autocrine, paracrine, or/and endocrine manner. EVs are released by the cells of the central nervous system (CNS), including neurons, astrocytes, oligodendrocytes and microglia, and have the ability to cross the blood brain barrier (BBB) and enter the systemic circulation. Neuroendocrine cells are specialized neurons that secrete hormones directly into blood vessels, such as the hypophyseal portal system or the systemic circulation, a process that allows neuroendocrine integration to take place. In mammals, neuroendocrine cells are widely distributed throughout various anatomic compartments, with the hypothalamus being a central neuroendocrine integrator. The hypothalamus is a key part of the Stress System (SS), a highly conserved neuronal/neuroendocrine system aiming at maintaining systemic homeostasis, when the latter is threatened by various stressors. The central parts of the SS are the interconnected hypothalamic Corticotropin-releasing Hormone (CRH) and the brainstem Locus Caeruleus-Norepinephrine (LC-NE) systems, while their peripheral parts are respectively the pituitary-adrenal axis, and the sympathetic nervous/sympatho-adrenomedullary systems (SNS-SAM) as well as components of the parasympathetic system (PNS). During stress, multiple CNS loci show plasticity and undergo remodeling, partly mediated by increased glutamatergic and noradrenergic activity, and actions of cytokines and glucocorticoids (GCs), all regulated by the interaction of the hypothalamic-pituitary-adrenal (HPA) axis and the LC-NE/SNS-SAM systems. In addition, there are peripheral changes due to the increased secretion of stress hormones and pro-inflammatory cytokines in the context of stress-related systemic (para)inflammation.