Receptor-mediated angiotensin II transcytosis by brain microvessel endothelial cells

Associations of renin-angiotensin system inhibitor use with brain insulin signaling and neuropathology

OBJECTIVE

To examine the associations of renin-angiotensin system (RAS) inhibitor use with postmortem brain insulin signaling and neuropathology.

METHODS

Among Religious Orders Study participants, 150 deceased and autopsied older individuals (75 with diabetes matched to 75 without by age at death, sex, and education) had measurements of insulin receptor substrate-1 (IRS-1) and RAC-alpha serine/threonine protein kinase (AKT1) collected in the prefrontal cortex using ELISA and immunohistochemistry. Alzheimer's disease (AD), brain infarcts, and cerebral vessel pathology data were assessed by systematic neuropathologic evaluations. RAS inhibitor use was determined based on visual inspection of medication containers during study visits. The associations of RAS inhibitor use with brain insulin signaling measures and neuropathology were examined using adjusted regression analyses.

RESULTS

Of the 90 RAS inhibitor users (54 with diabetes), 65 had used only angiotensin-converting enzyme inhibitors, 11 only angiotensin II receptor blockers, and 14 used both. RAS inhibitor use was associated with lower pT308AKT1/total AKT1, but not with pS307IRS-1/total IRS-1 or the density of cells stained positive for pS616 IRS-1. RAS inhibitor use was not associated with the level of global AD pathology or amyloid beta burden, but it was associated with a lower tau-neurofibrillary tangle density. Additionally, we found a significant interaction between diabetes and RAS inhibitors on tangle density. Furthermore, AKT1 phosphorylation partially mediated the association of RAS inhibitor use with tau tangle density. Lastly, RAS inhibitor use was associated with more atherosclerosis, but not with other cerebral blood vessel pathologies or cerebral infarcts.

INTERPRETATION

Late-life RAS inhibitor use may be associated with lower brain AKT1 phosphorylation and fewer neurofibrillary tangles.

Genetic Ablation of Prorenin Receptor in the Rostral Ventrolateral Medulla Influences Blood Pressure and Hydromineral Balance in Deoxycorticosterone-Salt Hypertension

Non-enzymatic activation of renin via its interaction with prorenin receptor (PRR) has been proposed as a key mechanism of local renin-angiotensin system (RAS) activation. The presence of renin and angiotensinogen has been reported in the rostral ventrolateral medulla (RVLM). Overactivation of bulbospinal neurons in the RVLM is linked to hypertension (HTN). Previous studies have shown that the brain RAS plays a role in the pathogenesis of the deoxycorticosterone (DOCA)-salt HTN model. Thus, we hypothesized that PRR in the RVLM is involved in the local activation of the RAS, facilitating the development of DOCA-salt HTN. Selective PRR ablation targeting the RVLM (PRRRVLM-Null mice) resulted in an unexpected sex-dependent and biphasic phenotype in DOCA-salt HTN. That is, PRRRVLM-Null females (but not males) exhibited a significant delay in achieving maximal pressor responses during the initial stage of DOCA-salt HTN. Female PRRRVLM-Null subsequently showed exacerbated DOCA-salt-induced pressor responses during the "maintenance" phase with a maximal peak at 13 d on DOCA-salt. This exacerbated response was associated with an increased sympathetic drive to the resistance arterioles and the kidney, exacerbated fluid and sodium intake and output in response to DOCA-salt, and induced mobilization of fluids from the intracellular to extracellular space concomitant with elevated vasopressin. Ablation of PRR suppressed genes involved in RAS activation and catecholamine synthesis in the RVLM but also induced expression of genes involved in inflammatory responses. This study illustrates complex and sex-dependent roles of PRR in the neural control of BP and hydromineral balance through autonomic and neuroendocrine systems. Graphical abstract.

No evidence for brain renin-angiotensin system activation during DOCA-salt hypertension

Brain renin-angiotensin system (RAS) activation is thought to mediate deoxycorticosterone acetate (DOCA)-salt hypertension, an animal model for human primary hyperaldosteronism. Here, we determined whether brainstem angiotensin II is generated from locally synthesized angiotensinogen and mediates DOCA-salt hypertension. To this end, chronic DOCA-salt-hypertensive rats were treated with liver-directed siRNA targeted to angiotensinogen, the angiotensin II type 1 receptor antagonist valsartan, or the mineralocorticoid receptor antagonist spironolactone (n = 6-8/group). We quantified circulating angiotensinogen and renin by enzyme-kinetic assay, tissue angiotensinogen by Western blotting, and angiotensin metabolites by LC-MS/MS. In rats without DOCA-salt, circulating angiotensin II was detected in all rats, whereas brainstem angiotensin II was detected in 5 out of 7 rats. DOCA-salt increased mean arterial pressure by 19 ± 1 mmHg and suppressed circulating renin and angiotensin II by >90%, while brainstem angiotensin II became undetectable in 5 out of 7 rats (<6 fmol/g). Gene silencing of liver angiotensinogen using siRNA lowered circulating angiotensinogen by 97 ± 0.3%, and made brainstem angiotensin II undetectable in all rats (P<0.05 vs. non-DOCA-salt), although brainstem angiotensinogen remained intact. As expected for this model, neither siRNA nor valsartan attenuated the hypertensive response to DOCA-salt, whereas spironolactone normalized blood pressure and restored brain angiotensin II together with circulating renin and angiotensin II. In conclusion, despite local synthesis of angiotensinogen in the brain, brain angiotensin II depended on circulating angiotensinogen. That DOCA-salt suppressed circulating and brain angiotensin II in parallel, while spironolactone simultaneously increased brain angiotensin II and lowered blood pressure, indicates that DOCA-salt hypertension is not mediated by brain RAS activation.

Perivascular macrophages mediate the neurovascular and cognitive dysfunction associated with hypertension

Hypertension is a leading risk factor for dementia, but the mechanisms underlying its damaging effects on the brain are poorly understood. Due to a lack of energy reserves, the brain relies on continuous delivery of blood flow to its active regions in accordance with their dynamic metabolic needs. Hypertension disrupts these vital regulatory mechanisms, leading to the neuronal dysfunction and damage underlying cognitive impairment. Elucidating the cellular bases of these impairments is essential for developing new therapies. Perivascular macrophages (PVMs) represent a distinct population of resident brain macrophages that serves key homeostatic roles but also has the potential to generate large amounts of reactive oxygen species (ROS). Here, we report that PVMs are critical in driving the alterations in neurovascular regulation and attendant cognitive impairment in mouse models of hypertension. This effect was mediated by an increase in blood-brain barrier permeability that allowed angiotensin II to enter the perivascular space and activate angiotensin type 1 receptors in PVMs, leading to production of ROS through the superoxide-producing enzyme NOX2. These findings unveil a pathogenic role of PVMs in the neurovascular and cognitive dysfunction associated with hypertension and identify these cells as a putative therapeutic target for diseases associated with cerebrovascular oxidative stress.

The Prorenin and (Pro)renin Receptor: New Players in the Brain Renin-Angiotensin System?

It is well known that the brain renin-angiotensin (RAS) system plays an essential role in the development of hypertension, mainly through the modulation of autonomic activities and vasopressin release. However, how the brain synthesizes angiotensin (Ang) II has been a debate for decades, largely due to the low renin activity. This paper first describes the expression of the vasoconstrictive arm of RAS components in the brain as well as their physiological and pathophysiological significance. It then focus on the (pro)renin receptor (PRR), a newly discovered component of the RAS which has a high level in the brain. We review the role of prorenin and PRR in peripheral organs and emphasize the involvement of brain PRR in the pathogenesis of hypertension. Some future perspectives in PRR research are heighted with respect to novel therapeutic target for the treatment of hypertension and other cardiovascular diseases.

Renin angiotensin system gene polymorphisms and cerebral blood flow regulation: the MOBILIZE Boston study

BACKGROUND AND PURPOSE

Our objective was to investigate the associations between polymorphisms in representative genes of the renin angiotensin system with measures of cerebral blood flow regulation in older adults.

METHODS

Participants in this analysis were white subjects (n=335) in the MOBILIZE Boston study (Maintenance of Balance, Independent Living, Intellect, and Zest in the Elderly of Boston), an observational study of community-dwelling elders who underwent transcranial Doppler while sitting and standing and during hypercapnea and hypocapnea. Autoregulation phenotype was the change in cerebrovascular resistance from sit to stand. Vasoreactivity phenotype was the slope of the change in cerebrovascular conductance versus change in end-tidal CO2. A total of 33 tagged single nucleotide polymorphisms were selected in the angiotensinogen gene, the angiotensin converting enzyme gene, and the angiotensin receptor gene. Regression analyses adjusted for age, gender, body mass index, mean arterial blood pressure, stroke, and use of antihypertensives were conducted for each single nucleotide polymorphism and outcome. Bonferroni corrections were used to adjust P values for multiple testing.

RESULTS

In the angiotensinogen gene, only the rs699 single nucleotide polymorphism was associated with vasoreactivity after Bonferroni correction (P=0.00028). Homozygous carriers of the CC genotype of this single nucleotide polymorphism had lower vasoreactivity compared with the CT or TT genotypes. There were no significant associations with autoregulation measures. None of the single nucleotide polymorphisms in the other genes were associated with our phenotypes.

CONCLUSION

This analysis suggests that the angiotensinogen gene may be involved in vasoreactivity independent of blood pressure. Larger studies are needed to confirm the role of this gene in cerebrovascular health and aging.

Angiotensin II modulates BBB permeability via activation of the AT(1) receptor in brain endothelial cells

Hypertensive encephalopathy occurs when acute changes in blood pressure cause breakdown of the blood-brain barrier (BBB). Angiotensin II (Ang II) plays a role in this pathophysiology. We determined whether Ang II directly regulates endothelial cell function at the BBB. In BBB microvessel endothelial cells (MECs), the Ang II (100 nmol/L; 0 to 6 h) effects on permeability to (125)I-albumin and transendothelial electrical resistance (TEER) were assessed. Angiotensin II (100 nmol/L) caused significant time-dependent changes in both (125)I-albumin permeability (25%) at 2 h and TEER (-8.87 Omega x cm(2)) at 6 h. Next, MECs were pretreated with the Ang II type 1 (AT(1)) receptor blocker telmisartan (1 micromol/L) or the Ang II type 2 (AT(2)) receptor blocker PD123,319 (1 micromol/L) followed by treatment with Ang II (100 nm). Telmisartan completely inhibited the Ang II-induced increase in (125)I-albumin permeability in MECs whereas PD123,319 had no effect. Using western blot analysis, we showed that MECs express AT(1) receptors but not AT(2) receptors. Treatment with Ang II (100 nmol/L; 0 to 6 h) also increased total protein kinase C activity. In contrast, Ang II had no effect on the expression of occludin, claudin 5, or actin. These results show that Ang II directly modulates transcytotic and paracellular permeability in BBB endothelial cells and could contribute to the pathophysiology of hypertensive encephalopathy.

Self-regulation of agonist activity at the Y receptors

Neuropeptide Y (NPY) is one of the most abundant neuropeptides, and is likely to be present at nanomolar levels over extended periods in the synaptic space of many forebrain areas. This might be linked to an evolved generalized toning activity through a number of other peptide receptors that use C-terminally amidated agonists (with LHRH and orexin receptors and GIR as examples). However, the Y1 and Y2 receptors (which constitute the bulk of Y receptors active in the neural matrix) possess subnanomolar affinities that, at saturating NPY levels, could produce excessive signaling, as well as receptor losses via repeated endocytosis. The related Y4 receptor shows an even higher agonist affinity, and faces the same problem in visceral and neural locations accessible to pancreatic polypeptide (PP). An examination of agonist peptide interaction with Y receptors shows that Y1 and Y4 receptors in particular (as located on either the intact cells, or on particulates derived from various cell types) develop a blockade dependent on ligand concentration, with the blocking ranks of [NPY]>>[peptide YY] (PYY) for the Y1, and [human PP]>>>[PYY-related Y4 agonist] for the Y4 receptor. This blockade is also echoed in a concentration-related reduction in biological activity of primary agonists (NPY and PP), resembling a partial agonism, and is influenced especially by the allosteric interactivity of agonists. With the Y2 receptor, the blocking by agonists is less pronounced, but the signaling by NPY-related peptides is apparently less than with PYY-related agonists. The extended occupancy and self-attenuation of primary agonist activity at Y receptors could represent an evolutionary solution contributing to a balancing of metabolic signaling, agonist clearance and receptor conservation.

Tobacco smoke chemicals attenuate brain-to-blood potassium transport mediated by the Na,K,2Cl-cotransporter during hypoxia-reoxygenation

Smoking tobacco, including cigarettes, has been associated with an increased incidence and relative risk for cerebral infarction in both men and women. Recently, we have shown that nicotine and cotinine attenuate abluminal (brain facing) K(+) uptake mediated by the Na,K,2Cl-cotransporter (NKCC) in bovine brain microvessel endothelial cells (BBMECs) after hypoxic/aglycemic exposure (stroke conditions). The purpose of the current study was to explore the effects of nicotine and tobacco smoke chemicals on K(+) movement through the blood-brain barrier during both hypoxia/aglycemia and reoxygenation. BBMECs were exposed to nicotine/cotinine, nicotine-containing cigarette smoke extract (N-CSE), or nicotine-free cigarette smoke extract (NF-CSE) in quantities designed to mimic plasma concentrations of smokers. Stroke conditions were mimicked in vitro in BBMECs through 6 h of hypoxia/aglycemia with or without 12 h of reoxygenation, after which NKCC-mediated K(+) uptake and paracellular integrity were measured with (86)Rb and [(14)C]sucrose, respectively. In addition, K(+) concentrations in brain extracellular fluid were estimated in (86)Rb-injected rats that were administered nicotine, N-CSE, or NF-CSE and on whom global ischemia/reperfusion by in vivo four-vessel occlusion was performed. Both in vitro and in vivo paradigms showed nicotine, the major alkaloid present in tobacco smoke, to be the determining factor of an inhibited response of abluminal NKCC in BBMECs during and after stroke conditions. This was measured as a decrease in abluminal brain endothelial cell NKCC activity and as an increase in brain extracellular K(+) concentration measured as the brain extracellular fluid (86)Rb/plasma ratio after in vivo four-vessel occlusion with reperfusion.

Angiotensin II AT1 receptor blockade abolishes brain microvascular inflammation and heat shock protein responses in hypertensive rats

Endothelial dysfunction and inflammation enhance vulnerability to hypertensive brain damage. To explore the participation of Angiotensin II (Ang II) in the mechanism of vulnerability to cerebral ischemia during hypertension, we examined the expression of inflammatory factors and the heat shock protein (HSP) response in cerebral microvessels from spontaneously hypertensive rats and their normotensive controls, Wistar Kyoto rats. We treated animals with vehicle or the Ang II AT(1) receptor antagonist candesartan, 0.3 mg/kg/day, via subcutaneously implanted osmotic minipumps for 4 weeks. Spontaneously hypertensive rats expressed higher Angiotensin II AT(1) receptor protein and mRNA than normotensive controls. Candesartan decreased the macrophage infiltration and reversed the enhanced tumor necrosis factor-alpha and interleukin-1beta mRNA and nuclear factor-kappaB in microvessels in hypertensive rats. The transcription of many HSP family genes, including HSP60, HSP70 and HSP90, and heat shock factor-1 was higher in hypertensive rats and was downregulated by AT(1) receptor blockade. Our results suggest a proinflammatory action of Ang II through AT(1) receptor stimulation in cerebral microvessels during hypertension, and very potent antiinflammatory effects of the Ang II AT(1) receptor antagonist. These compounds might be considered as potential therapeutic agents against ischemic and inflammatory diseases of the brain.

In vivo and in vitro assessment of baseline blood-brain barrier parameters in the presence of novel nanoparticles

PURPOSE

Nanoparticles have advantage as CNS drug delivery vehicles given they disguise drug permeation limiting characteristics. Conflicting toxicological data, however, is published with regard to blood-brain barrier integrity and gross mortality.

METHODS

To address this issue two novel nanoparticle types: "emulsifying wax/Brij 78" and "Brij 72/Tween 80 nanoparticles were evaluated in vivo for effect on cerebral perfusion flow, barrier integrity, and permeability using the in situ brain perfusion technique. Additional evaluation was completed in vitro using bovine brain microvessel endothelial cells for effect on integrity, permeability, cationic transport interactions, and tight junction protein expression.

RESULTS

In the presence of either nanoparticle formulation, no overall significant differences were observed for cerebral perfusion flow in vivo. Furthermore, observed in vitro and in vivo data showed no statistical changes in barrier integrity, membrane permeability, or facilitated choline transport. Western blot analyses of occludin and claudin-1 confirmed no protein expression changes with incubation of either nanoparticle.

CONCLUSIONS

The nanoparticle formulations appear to have no effect on primary BBB parameters in established in vitro and in vivo blood-brain barrier models.

Blood-brain barrier transport of a novel micro 1-specific opioid peptide, H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA)

The purpose of this study was to clarify the mechanism of the blood-brain barrier (BBB) transport of H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA), which is a novel dermorphin analog with high affinity for the micro 1-opioid receptor. The in vivo BBB permeation influx rate of [125I]TAPA after an i.v. bolus injection (7.3 pmol/g body weight) into mice was estimated to be 0.265 +/- 0.025 microL/(min.g of brain). The influx rate of [125I]TAPA was reduced 70% by the coadministration of unlabeled TAPA (33 nmol/g of brain), suggesting the existence of a specific transport system for TAPA at the BBB. In order to elucidate the BBB transport mechanism of TAPA, a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4) was used as an in vitro model of the BBB. The acid-resistant binding of [125I]TAPA, which represents the internalization of the peptide into cells, was temperature- and concentration-dependent with a half-saturation constant of 10.0 +/- 1.7 microm. The acid-resistant binding of TAPA was significantly inhibited by 2,4-dinitrophenol, dansylcadaverine (an endocytosis inhibitor) and poly-l-lysine and protamine (polycations). These results suggest that TAPA is transported through the BBB by adsorptive-mediated endocytosis, which is triggered by binding of the peptide to negatively charged sites on the surface of brain capillary endothelial cells. Blood-brain barrier transport via adsorptive-mediated endocytosis plays a key role in the expression of the potent opioid activity of TAPA in the CNS.

Nicotine and cotinine modulate cerebral microvascular permeability and protein expression of ZO-1 through nicotinic acetylcholine receptors expressed on brain endothelial cells

The blood-brain barrier (BBB) adapts to a variety of pathological processes. Little is known about the effects of nicotine exposure on BBB function and the ability to adapt to stroke conditions. We have demonstrated, using a well-characterized in vitro BBB model, bovine brain microvessel endothelial cells (BBMEC) model, that nicotine and its major metabolite, cotinine, modulate BBB integrity by opening the paracellular route of solute entry into the brain. Additionally, nicotine and cotinine together increase the permeability change observed after 6 h of hypoxia/aglycemia, an in vitro model of stroke. This has important implications for how the BBB initially adapts to stroke in an environment that is previously exposed to nicotine. Nicotine and cotinine exposure also resulted in reduced ZO-1 immunoreactivity (tight junctional protein) that occurred in a time-dependent manner. Interestingly, attenuation of bovine brain microvessel endothelial cell (BBMEC) ZO-1 protein expression was reversed using 10 nM BGT, an alpha7 nicotinic acetycholine receptor (nAChR) antagonist, suggesting that the effects of nicotine on BBMEC protein expression of ZO-1 protein are mediated by nAChR expressed on brain endothelial cells. In addition to alpha7, we found that BBMEC also contain positive immunoreactivity for the alpha3, alpha5, beta2, beta3 nAChR subunit. Both alpha7 and beta2 nAChR subunit protein levels decreased with prior nicotine and cotinine exposure. These data provide evidence that nicotine and cotinine alter BBB permeability and tight junctional protein expression of ZO-1, thereby altering the BBB response to stroke conditions. These changes in brain endothelial cell paracellular permeability are believed to be associated with nicotine binding to nAChRs present at the BBB.

Amyloid peptide toxicity and microtubule-stabilizing drugs

Based on microtubule (MT) disruption observed in primary neurons exposed to fibrillar amyloid peptides (A beta), we tested the potential protective effect of MT-stabilizing drugs such as Taxol against A beta-induced disruption of the cytoskeleton. Although Taxol was strongly protective, the fact that it does not cross the blood brain barrier (BBB) led us to synthesize and test other agents with MT-stabilizing properties and possible penetration into the brain. Our studies have thus far demonstrated that several MT-stabilizing agents, including some with structures quite different from that of Taxol, showed significant protective effects. However, not all agents that promoted MT-assembly were protective, suggesting additional mechanisms are involved in the actions of the drugs. A small number of neuroprotective compounds appear to have potential to enter the brain and thus might be tested to see if they slow progression of neurodegeneration in an appropriate animal model of Alzheimer's disease.

The chemokine receptor CCR2 mediates the binding and internalization of monocyte chemoattractant protein-1 along brain microvessels

Previous results from this laboratory revealed the presence of high-affinity saturable binding sites for monocyte chemoattractant protein-1 (MCP-1) along human brain microvessels (Andjelkovic et al., 1999; Andjelkovic and Pachter, 2000), which suggested that CC chemokine receptor 2 (CCR2), the recognized receptor for this chemokine, was expressed by the brain microvascular endothelium. To test the role of CCR2 directly in mediating MCP-1 interactions with the brain microvasculature, we assessed MCP-1 binding activity in murine brain microvessels isolated from wild-type mice and from CCR2 (-/-) mice engineered to lack this receptor. Results demonstrate that MCP-1 binding is greatly attenuated in microvessels prepared from CCR2 (-/-) mice compared with wild-type controls. Moreover, microvessels from wild-type mice exhibited MCP-1-induced downmodulation in MCP-1 binding and a recovery of binding activity that was not dependent on de novo protein synthesis. Furthermore, MCP-1 was shown to be internalized within wild-type microvessels, but not within microvessels obtained from CCR2 (-/-) mice, additionally demonstrating that CCR2 is obligatory for MCP-1 endocytosis. Last, internalization of MCP-1, but not transferrin, was observed to be inhibited by disruption of caveolae. Internalized MCP-1 also colocalized at some sites with caveolin-1, a major protein of caveolae, implying that this chemokine is endocytosed, in part, via nonclathrin-coated vesicles. These results prompt consideration that MCP-1 signals may be relayed across the blood-brain barrier by highly specialized interactions of this chemokine with its cognate receptor, CCR2, along brain microvascular endothelial cells.

Investigation of substance P transport across the blood-brain barrier

The transport of substance P (SP) was investigated using the bovine brain microvessel endothelial cell culture model of the blood-brain barrier (BBB). The samples were derivatized precolumn with naphthalene dialdehyde, then analyzed by cyclodextrin-modified micellar electrokinetic chromatography with laser-induced fluorescence detection. SP crossed the BBB in both the apical-to-basolateral and basolateral-to-apical directions through an active transport mechanism. The transport of SP from the apical side was demonstrated to be via transcytosis. The N-terminal (SP(1-4)) and C-terminal (SP(3-11)) fragments were also found to permeate the BBB from the apical side.

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Stroke: development, prevention and treatment with peptidase inhibitors

Stroke is the leading cause of long-term disability in the United States and affects more people than any other neurologic disorder. Hypertension is a major risk factor associated with stroke. Several anti-hypertensive agents have been used to treat chronic hypertension to reduce the morbidity and mortality of stroke. Previous experimental studies have shown reduced stroke mortality with angiotensin-converting enzyme (ACE) inhibitors. This review discusses the development of stroke and potential use of ACE inhibitors in prevention and treatment of this disease. Furthermore, this review focuses on current investigations aimed at cellular mechanisms involved in stroke-induced microvascular alterations.

Blood-brain barrier transport of 125I-labeled basic fibroblast growth factor

PURPOSE

This study was carried out to examine the blood-brain barrier (BBB) transport of human basic fibroblast growth factor (bFGF) and investigate its mechanism.

METHODS

The BBB transport of 125I-bFGF was measured by several in vivo methods including intravenous administration, in situ internal carotid artery perfusion, and intracerebral microinjection. The in vitro binding of 125I-bFGF was characterized using freshly prepared bovine brain capillaries.

RESULTS

The distribution volume of 125I-bFGF in the postvascular supernatant increased with the perfusion time, and exceeded the space occupied by the brain microvasculature and its trichloroacetic acid (TCA) precipitability was more than 90%. 125I-bFGF avidly bound to isolated bovine brain capillaries with a Bmax of 206 +/- 48 pmol/mg protein, and a Kd of 36.5 +/- 15.7 nM. This binding was significantly inhibited by unlabeled bFGF and heparin in a concentration-dependent manner. The cationic peptides, protamine and poly-L-lysine (each 300 microM), produced over 85% inhibition of 125I-bFGF binding to brain capillaries. Furthermore, glycosaminoglycans with a sulfate residue, chondroitin sulfate B and C (each 10 microg/mL) also inhibited the binding of 125I-bFGF The in vivo transcytosis of 125I-bFGF from the luminal side to the brain was also inhibited by the presence of heparin (10 microg/mL) and poly-L-lysine (300 microM), whereas neither hyaruronic acid (10 microg/mL) nor insulin (10 microM) had any effect. In addition to these results, the brain efflux index method was used to confirm that the transcytosis of 125I-bFGF from brain to blood across the BBB was negligible.

CONCLUSIONS

These results suggest that 125I-bFGF is transported across the BBB, possibly by an adsorptive-mediated transcytosis mechanism that is triggered by binding to negatively charged species on the luminal membrane surface of the brain microvasculature, such as heparan sulfate proteoglycans.

Transport of the delta-opioid receptor agonist [D-penicillamine2,5] enkephalin across the blood-brain barrier involves transcytosis1

The delta opioid receptor antagonist [D-penicillamine2,5]enkephalin (DPDPE) is an enzymatically stable peptide analogue of Met-enkephalin. DPDPE uses a saturable transport mechanism to cross the blood-brain barrier (BBB), though the exact mechanism is not fully understood. The aim of the present study was to identify the mechanism by which DPDPE enters the brain. The effect of phenylarsine oxide (PAO), an endocytosis inhibitor, on the transport of [3H]DPDPE was investigated using both in vitro and in situ transport studies. Two in vitro models of the BBB utilizing primary bovine brain microvascular endothelial cells (BBMEC) were studied. [3H]DPDPE permeability across monolayers of BBMEC grown on polycarbonate filters was studied. PAO significantly reduced the permeability of [3H]DPDPE across the monolayer. PAO also reduced the uptake of [3H]DPDPE into BBMEC cells, without affecting binding to the cells. The in situ perfusion model of the BBB was also studied, PAO reduced DPDPE uptake by the brain in a dose-dependent manner. These studies indicate that DPDPE enters the brain via an energy-dependent transcytotic mechanism.

AT1 receptors mediate angiotensin II uptake and transport by bovine brain microvessel endothelial cells in primary culture

The endothelial lining of the blood-brain barrier tightly controls the distribution of peptide hormones between the central nervous system and the circulation. By using primary cultures of brain microvessel endothelial cells, an in vitro model of the blood-brain barrier, we report here the uptake and transport of the octapeptide angiotensin II by a specific receptor population. With the angiotensin II antagonists losartan (AT1 specific) and PD 123,319 (AT2 specific), we showed that both the uptake and transport of angiotensin II were mediated by the AT1 receptor. Western blot analysis confirmed the existence of the AT1 receptor in our cell-culture model. Rhodamine 123 studies also suggested that both angiotensin II antagonists, but not angiotensin II, were substrates for the P-glycoprotein efflux system, thus restricting the transport of these compounds. These results suggest an AT1 receptor mediates uptake and transport of angiotensin II at the blood-brain barrier and may contribute to the regulation of cerebrovascular levels of the peptide.