Endothelial vesicles in the blood-brain barrier: are they related to permeability?

Serum growth factors and neuroprotective surveillance: focus on IGF-1

The adult brain requires a constant trophic input for appropriate function. Although the main source of trophic factors for mature neurons is considered to arise locally from glial cells and synaptic partners, recent evidence suggests that hormonal-like influences from distant sources may also be important. These include not only relatively well-characterized steroid hormones that cross the brain barriers, but also blood-borne protein growth factors able to cross the barriers and exert unexpected, albeit specific, trophic actions in diverse brain areas. Insulin-like growth factor I (IGF-I) is until now the serum neurotrophic factor whose actions on the adult brain are best-characterized. This is because IGF-I has been known for many years to be present in serum, whereas the presence in the circulation of other more classical neurotrophic factors has only recently been recognized. Thus, new evidence strongly suggests that IGF-I, and other blood-borne neurotrophic factors such as Fibroblast Growth Factor (FGF-2) or the neurotrophins, exert a tonic trophic input on brain cells, providing a mechanism for what we may refer to as neuroprotective surveillance. Protective surveillance includes "first-line" defense mechanisms ranging from blockade of neuronal death after a wide variety of cellular insults to upregulation of neurogenesis when defenses against neuronal death are overcome. Most importantly, surveillance should also encompass modulation of homeostatic mechanisms to prevent neuronal derangement. These will include modulation of basic cellular processes such as metabolic demands and maintainance of cell-membrane potential as well as more complex processes such as regulation of neuronal plasticity to keep neurons able to respond to constantly changing functional demands.

Inflammatory mediators and modulation of blood-brain barrier permeability

1. Unlike some interfaces between the blood and the nervous system (e.g., nerve perineurium), the brain endothelium forming the blood-brain barrier can be modulated by a range of inflammatory mediators. The mechanisms underlying this modulation are reviewed, and the implications for therapy of the brain discussed. 2. Methods for measuring blood-brain barrier permeability in situ include the use of radiolabeled tracers in parenchymal vessels and measurements of transendothelial resistance and rate of loss of fluorescent dye in single pial microvessels. In vitro studies on culture models provide details of the signal transduction mechanisms involved. 3. Routes for penetration of polar solutes across the brain endothelium include the paracellular tight junctional pathway (usually very tight) and vesicular mechanisms. Inflammatory mediators have been reported to influence both pathways, but the clearest evidence is for modulation of tight junctions. 4. In addition to the brain endothelium, cell types involved in inflammatory reactions include several closely associated cells including pericytes, astrocytes, smooth muscle, microglia, mast cells, and neurons. In situ it is often difficult to identify the site of action of a vasoactive agent. In vitro models of brain endothelium are experimentally simpler but may also lack important features generated in situ by cell:cell interaction (e.g. induction, signaling). 5. Many inflammatory agents increase both endothelial permeability and vessel diameter, together contributing to significant leak across the blood-brain barrier and cerebral edema. This review concentrates on changes in endothelial permeability by focusing on studies in which changes in vessel diameter are minimized. 6. Bradykinin (Bk) increases blood-brain barrier permeability by acting on B2 receptors. The downstream events reported include elevation of [Ca2+]i, activation of phospholipase A2, release of arachidonic acid, and production of free radicals, with evidence that IL-1 beta potentiates the actions of Bk in ischemia. 7. Serotonin (5HT) has been reported to increase blood-brain barrier permeability in some but not all studies. Where barrier opening was seen, there was evidence for activation of 5-HT2 receptors and a calcium-dependent permeability increase. 8. Histamine is one of the few central nervous system neurotransmitters found to cause consistent blood-brain barrier opening. The earlier literature was unclear, but studies of pial vessels and cultured endothelium reveal increased permeability mediated by H2 receptors and elevation of [Ca2+]i and an H1 receptor-mediated reduction in permeability coupled to an elevation of cAMP. 9. Brain endothelial cells express nucleotide receptors for ATP, UTP, and ADP, with activation causing increased blood-brain barrier permeability. The effects are mediated predominantly via a P2U (P2Y2) G-protein-coupled receptor causing an elevation of [Ca2+]i; a P2Y1 receptor acting via inhibition of adenyl cyclase has been reported in some in vitro preparations. 10. Arachidonic acid is elevated in some neural pathologies and causes gross opening of the blood-brain barrier to large molecules including proteins. There is evidence that arachidonic acid acts via generation of free radicals in the course of its metabolism by cyclooxygenase and lipoxygenase pathways. 11. The mechanisms described reveal a range of interrelated pathways by which influences from the brain side or the blood side can modulate blood-brain barrier permeability. Knowledge of the mechanisms is already being exploited for deliberate opening of the blood-brain barrier for drug delivery to the brain, and the pathways capable of reducing permeability hold promise for therapeutic treatment of inflammation and cerebral edema.