Ultrastructural study on the interaction of insulin-albumin-gold complex with mouse brain microvascular endothelial cells

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

1. Macromolecules cross capillary walls via large vascular pores that are thought to be formed by plasmalemmal vesicles. Early hypotheses suggested that vesicles transferred plasma constituents across the endothelial wall either by a "shuttle" mechanism or by fusing to form transient patent channels for diffusion. Recent evidence shows that the transcytotic pathway involves both movement of vesicles within the cell and a series of fusions and fissions of the vesicular and cellular membranes. 2. The transfer of macromolecules across the capillary wall is highly specific and is mediated by receptors incorporated into specific membrane domains. Therefore, despite their morphological similarity, endothelial vesicles from heterogeneous populations in which the predominant receptor proteins incorporated in their membranes define the functions of individual vesicles. 3. Blood-brain barrier capillaries have very low permeabilities to most hydrophilic molecules. Their low permeability to macromolecules has been presumed to be due to an inhibition of the transcytotic mechanism, resulting in a low density of endothelial vesicles. 4. A comparison of vesicular densities and protein permeabilities in a number of vascular beds shows only a very weak correlation, therefore vesicle numbers alone cannot be used to predict permeability to macromolecules. 5. Blood-brain barrier capillaries are fully capable of transcytosing specific proteins, for example, insulin and transferrin, although the details are still somewhat controversial. 6. It has recently been shown that the albumin binding protein gp60 (also known as albondin), which facilitates the transcytosis of native albumin in other vascular beds, is virtually absent in brain capillaries. 7. It seems likely that the low blood-brain barrier permeability to macromolecules may be due to a low level of expression of specific receptors, rather than to an inhibition of the transcytosis mechanism.

Ultrastructural study on the interaction of native and cationized albumin-gold complexes with mouse brain microvascular endothelium

The main objective of this ultrastructural study was to gain insights into the cellular mechanisms responsible for the enhanced brain uptake of blood-borne cationized albumin observed by several authors utilizing quantitative methodology. Mice were injected intravenously or into the common carotid artery (in vivo experiments) or perfused in situ with solutions of native or cationized bovine serum albumin complexed with colloidal gold (BSA-G or cBSA-G respectively). The results indicate that: (1) the main drawbacks of in vivo experiments are very intense phagocytosis of the tracer particles by Kupffer cells located in the liver sinusoids and also escape of the tracer through capillaries of skeletal and heart muscles. This results in a rapid decline of the concentration and disappearance of the circulating tracer particles; (2) BSA-G and cBSA-G both in vivo (up to 30 min circulation) or after perfusion in situ (up to 15 min) do not cross the wall of brain microvessels representing the blood-brain barrier; (3) enhanced entrance of cationized albumin into the brain occurs through fenestrated endothelium of the capillaries located in the examined circumventricular organs (median eminence and neurohypophysis). Although BSA-G is also transported by these fenestrated capillaries, this process is evidently less intense than in the case of cBSA-G; (4) the enhanced passage of cBSA-G across fenestrated capillaries occurs mainly via vesicular transport (adsorptive transcytosis), through transendothelial channels and eventually through interendothelial junctional clefts; (5) the fenestrated capillaries of the choroid plexus appear to be less permeable for both tracers than those located in the other circumventricular organs.

Comparison of saturable transport and extracellular pathways in the passage of interleukin-1 alpha across the blood-brain barrier

Blood-borne cytokines enter the brain by transport across the blood-brain barrier (BBB) or by leakage through extracellular pathways at sites, such as circumventricular organs (CVOs), without a BBB. We used radioactively labeled albumin (T-Alb) to differentiate the relative contribution of transport and extracellular pathways to the passage of interleukin-1 alpha ([125I]IL-1 alpha) across the BBB. The major mechanism of entry for [125I]IL-1 alpha after intravenous (i.v.) injection was a saturable transport system with extracellular pathways accounting for only a small fraction of entry into brain. CVOs concentrated blood-borne [125I]IL-1 alpha in a saturable manner to a much greater extent than did the cerebral cortex and cerebellum, but accounted for less that 5% of total brain uptake. After intracerebroventricular (i.c.v.) injection, [125I]IL-1 alpha and T-Alb were concentrated in the CVOs, especially the median eminence, although CVOs contained less that 1% of the substances injected. Distribution after i.c.v. injection was largely due to diffusion and leakage through extracellular pathways. We conclude that after i.c.v. injection, leakage across extracellular pathways accounts for the small but concentrated amount of [125I]IL-1 alpha found in CVOs. After i.v. injection, transport across the BBB accounts for the majority of [125I]IL-1 alpha in brain.

Human insulin receptor monoclonal antibody undergoes high affinity binding to human brain capillaries in vitro and rapid transcytosis through the blood-brain barrier in vivo in the primate

PURPOSE

The ability of monoclonal antibodies against the human insulin receptor to undergo transcytosis through the blood-brain barrier (BBB) was examined in the present studies.

METHODS

Two murine monoclonal antibodies (MAb83-7 and MAb83-14) which bind different epitopes within the alpha-subunit of the human insulin receptor were examined using isolated human brain capillaries, frozen sections of primate brain, and in vivo pharmacokinetic studies in anesthetized Rhesus monkeys.

RESULTS

Both antibodies strongly illuminated capillary endothelium in immunocytochemical analysis of frozen sections of brain from Rhesus monkey but not squirrel monkey. Both monoclonal antibodies, in the iodinated forms, bound to human brain microvessels, although the binding and endocytosis of MAb83-14 was approximately 10-fold greater than MAb83-7. The active binding of MAb83-14 to the human insulin receptor was paralleled by a very high rate of transport of this antibody through the BBB in vivo in two anesthetized Rhesus monkeys. The BBB permeability-surface area (PS) product in neocortical gray matter was 5.4 +/- 0.6 microL/min/g, which is severalfold greater than previous estimates of the PS product for receptor-specific monoclonal antibody transport through the BBB. The brain delivery of MAb83-14 to the Rhesus monkey brain was high and 3.8 +/- 0.4% of the injected dose was delivered to 100 g of brain at 3 hours after a single intravenous injection. In contrast, there was no brain uptake of the mouse IgG2a isotype control antibody.

CONCLUSIONS

These studies demonstrate an unexpected high degree of transcytosis of a monoclonal antibody through the primate BBB in vivo.