Modulation of a glial blood-brain barrier

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Shiga toxin 1 induces on lipopolysaccharide-treated astrocytes the release of tumor necrosis factor-alpha that alter brain-like endothelium integrity

The hemolytic uremic syndrome (HUS) is characterized by hemolytic anemia, thrombocytopenia and renal dysfunction. The typical form of HUS is generally associated with infections by Gram-negative Shiga toxin (Stx)-producing Escherichia coli (STEC). Endothelial dysfunction induced by Stx is central, but bacterial lipopolysaccharide (LPS) and neutrophils (PMN) contribute to the pathophysiology. Although renal failure is characteristic of this syndrome, neurological complications occur in severe cases and is usually associated with death. Impaired blood-brain barrier (BBB) is associated with damage to cerebral endothelial cells (ECs) that comprise the BBB. Astrocytes (ASTs) are inflammatory cells in the brain and determine the BBB function. ASTs are in close proximity to ECs, hence the study of the effects of Stx1 and LPS on ASTs, and the influence of their response on ECs is essential. We have previously demonstrated that Stx1 and LPS induced activation of rat ASTs and the release of inflammatory factors such as TNF-α, nitric oxide and chemokines. Here, we demonstrate that rat ASTs-derived factors alter permeability of ECs with brain properties (HUVECd); suggesting that functional properties of BBB could also be affected. Additionally, these factors activate HUVECd and render them into a proagregant state promoting PMN and platelets adhesion. Moreover, these effects were dependent on ASTs secreted-TNF-α. Stx1 and LPS-induced ASTs response could influence brain ECs integrity and BBB function once Stx and factors associated to the STEC infection reach the brain parenchyma and therefore contribute to the development of the neuropathology observed in HUS.

Hemolytic-uremic syndrome (HUS) is generally caused by Shiga toxin (Stx)-producing Escherichia coli but bacterial lipopolysaccharide (LPS) and neutrophils (PMN) contribute to the pathophysiology. Acute renal failure is the main feature of HUS, but in severe cases, patients develop neurological complications, which are usually associated with death. Although the mechanisms of neurological damage remain uncertain, alterations/injury of brain endothelial cells (ECs) which constitute the blood-brain barrier (BBB) is clear. Astrocytes (ASTs) are inflammatory cells enclosing ECs and are responsible of the normal function of the barrier. We have recently demonstrated that Stx1, one of the most common types of Stx, induce an inflammatory response in LPS-treated ASTs. We then study the effects of factors released by ASTs in response to LPS and/or Stx1 on brain-like ECs. We demonstrate that Stx1 induces in LPS-treated ASTs the release of factors that alter brain properties in ECs, including the permeability; turning them more susceptible to Stx1 toxic effects. Furthermore, they activate ECs, neutrophils (PMN) and platelets and render ECs into a proagregant state promoting PMN and platelet adhesion. Our results suggest that ASTs could influence brain ECs integrity and BBB function once Stx in combination with bacterial factors reach the brain parenchyma.

Potassium fluxes across the blood brain barrier of the cockroach, Periplaneta americana

Potassium fluxes across the blood-brain barrier of the cockroach Periplaneta americana were measured using the scanning ion-selective microelectrode technique. In salines containing 15 mM or 25 mMK(+), an efflux of K(+) from the ganglia of isolated nerve cords was counterbalanced by an influx across the connectives. Metabolic inhibition with CN(-) resulted in an increase in K(+) efflux across both the ganglia and the connectives. Depletion of K(+) by chilling the nerve cords in K(+)-free saline was associated with subsequent K(+) influx across the connectives in K(+)-replete saline at room temperature. There were dramatic increases in K(+) efflux across both ganglia and connectives when the nerve cords were exposed to the pore-forming antibiotic amphotericin B. K(+) fluxes across the ventral nerve cord were also altered when paracellular leakage was augmented by transient exposure to 3M urea. K(+) efflux was reduced by the K(+) channel blockers Ba(2+) and tetraethylammonium or by exposure to Ca(2+)-free saline and K(+) efflux from the ganglia was increased by addition of ouabain to the bathing saline. The results provide direct support for a model proposing that K(+) is cycled through a current loop between the ganglia and the connectives and that both the Na(+)/K(+)-ATPase and K(+) channels are implicated in extracellular K(+) homeostasis within the central nervous system.

Cyclical expression of Na+/K+-ATPase in the visual system of Drosophila melanogaster

In the first (lamina) and second (medulla) optic neuropils of Drosophila melanogaster, sodium pump subunit expression changes during the day and night, controlled by a circadian clock. We examined alpha-subunit expression from the intensity of immunolabeling. For the beta-subunit, encoded by Nervana 2 (Nrv2), we used Nrv2-GAL4 to drive expression of GFP, and measured the resultant fluorescence in whole heads and specific optic lobe cells. All optic neuropils express the alpha-subunit, highest at the beginning of night in both lamina and medulla in day/night condition and the oscillation was maintained in constant darkness. This rhythm was lacking in the clock arrhythmic per(0) mutant. GFP driven by Nrv2 was mostly detected in glial cells, mainly in the medulla. There, GFP expression occurs in medulla neuropil glia (MNGl), which express the clock gene per, and which closely contact the terminals of clock neurons immunoreactive to pigment dispersing factor. GFP fluorescence exhibited circadian oscillation in whole heads from Nrv2-GAL4+UAS-S65T-GFP flies, although significant GFP oscillations were lacking in MNGl, as they were for both subunit mRNAs in whole-head homogenates. In the dissected brain tissues, however, the mRNA of the alpha-subunit showed a robust daily rhythm in concentration changes while changes in the beta-subunit mRNA were weaker and not statistically significant. Thus in the brain, the genes for the sodium pump subunits, at least the one encoding the alpha-subunit, seem to be clock-controlled and the abundance of their corresponding proteins mirrors daily changes in mRNA, showing cyclical accumulation in cells.

The development of the blood-brain barrier in alcohol-exposed rats

Circulating horseradish peroxidase (HRP) was used as a tracer to determine if the blood-brain barrier to protein was altered by dietary prenatal alcohol exposure. Animals were prepared for light microscopic visualization of HRP after HRP infusion on gestational days 16, 18, 20, 22 and postnatal day 4. There was no consistent evidence of HRP leakage through the BBB in the alcohol-exposed animals compared to control animals. Capillary endothelial cells and perivascular astrocytic endfeet were morphologically characterized by electron microscopy in rat optic nerve and cerebellum following dietary prenatal and postnatal ethanol exposure. Photomontages of optic nerve capillaries from G20 and P5 animals and cerebellar capillaries from P15 animals were examined for evidences of effects of alcohol on the development of the capillaries and adjacent astroglial endfeet. There was no consistent evidence of any alcohol-induced effect that could indicate a disruption of the vessel, the endothelial tight junctions, the perivascular glial limiting membranes, or the extent of vascular ensheathment by astrocytic endfeet.