Internalization of targeted quantum dots by brain capillary endothelial cells in vivo

Receptors located on brain capillary endothelial cells forming the blood-brain barrier are the target of most brain drug delivery approaches. Yet, direct subcellular evidence of vectorized transport of nanoformulations into the brain is lacking. To resolve this question, quantum dots were conjugated to monoclonal antibodies (Ri7) targeting the murine transferrin receptor. Specific transferrin receptor-mediated endocytosis of Ri7-quantum dots was first confirmed in N2A and bEnd5 cells. After intravenous injection in mice, Ri7-quantum dots exhibited a fourfold higher volume of distribution in brain tissues, compared to controls. Immunofluorescence analysis showed that Ri7-quantum dots were sequestered throughout the cerebral vasculature 30 min, 1 h, and 4 h post injection, with a decline of signal intensity after 24 h. Transmission electron microscopic studies confirmed that Ri7-quantum dots were massively internalized by brain capillary endothelial cells, averaging 37 ± 4 Ri7-quantum dots/cell 1 h after injection. Most quantum dots within brain capillary endothelial cells were observed in small vesicles (58%), with a smaller proportion detected in tubular structures or in multivesicular bodies. Parenchymal penetration of Ri7-quantum dots was extremely low and comparable to control IgG. Our results show that systemically administered Ri7-quantum dots complexes undergo extensive endocytosis by brain capillary endothelial cells and open the door for novel therapeutic approaches based on brain endothelial cell drug delivery.

Proteomics and the blood-brain barrier: how recent findings help drug development

The drug discovery and development processes are divided into different stages separated by milestones to indicate that significant progress has been made and that certain criteria for target validation, hits, leads and candidate drugs have been met. Proteomics is a promising approach for the identification of protein targets and biochemical pathways involved in disease process and thus plays an important role in several stages of the drug development. The blood-brain barrier is considered as a major bottleneck when trying to target new compounds to treat neurodegenerative diseases. Based on the survey of recent findings and with a projection on expected improvements, this report attempt to address how proteomics participates in drug development.

Can insulin signaling pathways be targeted to transport Aβ out of the brain?

Although the causal role of Amyloid-β (Aβ) in Alzheimer’s disease (AD) is unclear, it is still reasonable to expect that lowering concentrations of Aβ in the brain may decrease the risk of developing the neurocognitive symptoms of the disease. Brain capillary endothelial cells forming the blood-brain barrier (BBB) express transporters regulating the efflux of Aβ out of the cerebral tissue. Age-related BBB dysfunctions, that have been identified in AD patients, might impair Aβ clearance from the brain. Thus, targeting BBB outward transport systems has been suggested as a way to stimulate the clearance of Aβ from the brain. Recent data indicate that the increase in soluble brain Aβ and behavioral impairments in 3×Tg-AD mice generated by months of intake of a high-fat diet can be acutely reversed by the administration of a single dose of insulin. A concomitant increase in plasma Aβ suggests that clearance from the brain through the BBB is a likely mechanism for this rapid effect of insulin. Here, we review how BBB insulin response pathways could be stimulated to decrease brain Aβ concentrations and improve cognitive performance, at least on the short term.

"You Shall Not Pass"-tight junctions of the blood brain barrier

The structure and function of the barrier layers restricting the free diffusion of substances between the central nervous system (brain and spinal cord) and the systemic circulation is of great medical interest as various pathological conditions often lead to their impairment. Excessive leakage of blood-borne molecules into the parenchyma and the concomitant fluctuations in the microenvironment following a transient breakdown of the blood-brain barrier (BBB) during ischemic/hypoxic conditions or because of an autoimmune disease are detrimental to the physiological functioning of nervous tissue. On the other hand, the treatment of neurological disorders is often hampered as only minimal amounts of therapeutic agents are able to penetrate a fully functional BBB or blood cerebrospinal fluid barrier. An in-depth understanding of the molecular machinery governing the establishment and maintenance of these barriers is necessary to develop rational strategies allowing a controlled delivery of appropriate drugs to the CNS. At the basis of such tissue barriers are intimate cell-cell contacts (zonulae occludentes, tight junctions) which are present in all polarized epithelia and endothelia. By creating a paracellular diffusion constraint TJs enable the vectorial transport across cell monolayers. More recent findings indicate that functional barriers are already established during development, protecting the fetal brain. As an understanding of the biogenesis of TJs might reveal the underlying mechanisms of barrier formation during ontogenic development numerous in vitro systems have been developed to study the assembly and disassembly of TJs. In addition, monitoring the stage-specific expression of TJ-associated proteins during development has brought much insight into the “developmental tightening” of tissue barriers. Over the last two decades a detailed molecular map of transmembrane and cytoplasmic TJ-proteins has been identified. These proteins not only form a cell-cell adhesion structure, but integrate various signaling pathways, thereby directly or indirectly impacting upon processes such as cell-cell adhesion, cytoskeletal rearrangement, and transcriptional control. This review will provide a brief overview on the establishment of the BBB during embryonic development in mammals and a detailed description of the ultrastructure, biogenesis, and molecular composition of epithelial and endothelial TJs will be given.

Involvement of insulin-degrading enzyme in insulin- and atrial natriuretic peptide-sensitive internalization of amyloid-β peptide in mouse brain capillary endothelial cells

Cerebral clearance of amyloid-β peptide (Aβ), which is implicated in Alzheimer's disease, involves elimination across the blood-brain barrier (BBB), and we previously showed that an insulin-sensitive process is involved in the case of Aβ1-40. The purpose of this study was to clarify the molecular mechanism of the insulin-sensitive Aβ1-40 elimination across mouse BBB. An in vivo cerebral microinjection study demonstrated that [125I]hAβ1-40 elimination from mouse brain was inhibited by human natriuretic peptide (hANP), and [125I]hANP elimination was inhibited by hAβ1-40, suggesting that hAβ1-40 and hANP share a common elimination process. Internalization of [125I]hAβ1-40 into cultured mouse brain capillary endothelial cells (TM-BBB4) was significantly inhibited by either insulin, hANP, other natriuretic peptides or insulin-degrading enzyme (IDE) inhibitors, but was not inhibited by phosphoramidon or thiorphan. Although we have reported the involvement of natriuretic peptide receptor C (Npr-C) in hANP internalization, cells stably expressing Npr-C internalized [125I]hANP but not [125I]hAβ1-40, suggesting that there is no direct interaction between Npr-C and hAβ1-40. IDE was detected in plasma membrane of TM-BBB4 cells, and internalization of [125I]hAβ1-40 by TM-BBB4 cells was reduced by IDE-targeted siRNAs. We conclude that elimination of hAβ1-40 from mouse brain across the BBB involves an insulin- and ANP-sensitive process, mediated by IDE expressed in brain capillary endothelial cells.

Quantitative targeted proteomics for understanding the blood-brain barrier: towards pharmacoproteomics

The blood-brain barrier (BBB) is formed by brain capillary endothelial cells linked together via complex tight junctions, and serves to prevent entry of drugs into the brain. Multiple transporters are expressed at the BBB, where they control exchange of materials between the circulating blood and brain interstitial fluid, thereby supporting and protecting the CNS. An understanding of the BBB is necessary for efficient development of CNS-acting drugs and to identify potential drug targets for treatment of CNS diseases. Quantitative targeted proteomics can provide detailed information on protein expression levels at the BBB. The present review highlights the latest applications of quantitative targeted proteomics in BBB research, specifically to evaluate species and in vivo-in vitro differences, and to reconstruct in vivo transport activity. Such a BBB quantitative proteomics approach can be considered as pharmacoproteomics.

T7 peptide-functionalized nanoparticles utilizing RNA interference for glioma dual targeting

Among all the malignant brain tumors, glioma is the deadliest and most common form with poor prognosis. Gene therapy is regarded as a promising way to halt the progress of the disease or even cure the tumor and RNA interference (RNAi) stands out. However, the existence of the blood-brain barrier (BBB) and blood tumor barrier (BTB) limits the delivery of these therapeutic genes. In this work, the delivery system targeting to the transferrin (Tf) receptor highly expressed on both BBB and glioma was successfully synthesized and would not compete with endogenous Tf. U87 cells stably express luciferase were employed here to simulate tumor and the RNAi experiments in vitro and in vivo validated that the gene silencing activity was 2.17-fold higher with the targeting ligand modification. The dual-targeting gene delivery system exhibits a series of advantages, such as high efficiency, low toxicity, stability and high transaction efficiency, which may provide new opportunities in RNAi therapeutics and nanomedicine of brain tumors.

Synergistic effects of neurons and astrocytes on the differentiation of brain capillary endothelial cells in culture

Brain capillary endothelial cells form a functional barrier between blood and brain, based on the existence of tight junctions that limit paracellular permeability. Occludin is one of the major transmembrane proteins of tight junctions and its peripheral localization gives indication of tight junction formation. We previously reported that RBE4.B cells (brain capillary endothelial cells), cultured on collagen IV, synthesize occludin and correctly localize it at the cell periphery only when cocultured with neurons. In the present study, we describe a three-cell type-culture system that allowed us to analyze the combined effects of neurons and astrocytes on differentiation of brain capillary endothelial cells in culture. In particular, we found that, in the presence of astrocytes, the neuron-induced synthesis and localization of occludin is precocious as compared to cells cocultured with neurons only.