Morphology of blood-brain interfaces

Poliovirus capsid protein VP3 can penetrate vascular endothelial cells

The poliovirus (PV) enters the central nervous system (CNS) via the bloodstream, suggesting the existence of a mechanism to cross the blood-brain barrier. Here, we report that PV capsid proteins (VP1 and VP3) can penetrate cells, with VP3 being more invasive. Two independent parts of VP3 are responsible for this function. Both peptides can penetrate human umbilical cord vascular endothelial cells, and one peptide of VP3 could also penetrate peripheral blood mononuclear cells. In an in vitro blood-brain barrier model using rat-derived astrocytes, pericytes, and endothelial cells, both peptides were observed to traverse from the blood side to the brain side at 6 h after administration. These results provide insights into the molecular mechanisms underlying PV invasion into the CNS.

Stem Cell Therapy and Thiamine Deficiency-Induced Brain Damage

Wernicke's encephalopathy (WE) is a major central nervous system disorder resulting from thiamine deficiency (TD) in which a number of brain regions can develop serious damage including the thalamus and inferior colliculus. Despite decades of research into the pathophysiology of TD and potential therapeutic interventions, little progress has been made regarding effective treatment following the development of brain lesions and its associated cognitive issues. Recent developments in our understanding of stem cells suggest they are capable of repairing damage and improving function in different maladys. This article puts forward the case for the potential use of stem cell treatment as a therapeutic strategy in WE by first examining the effects of TD on brain functional integrity and its consequences. The second half of the paper will address the future benefits of treating TD with these cells by focusing on their nature and their potential to effectively treat neurodegenerative diseases that share some overlapping pathophysiological features with TD. At the same time, some of the obstacles these cells will have to overcome in order to become a viable therapeutic strategy for treating this potentially life-threatening illness in humans will be highlighted.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Exposure of human cerebral microvascular endothelial cells hCMEC/D3 to laminar shear stress induces vascular protective responses

Endothelial cells (ECs) are constantly submitted in vivo to hemodynamical forces derived from the blood circulation, including shear stress (SS). ECs are able to detect SS and consequently adapt their phenotype, thus affecting many endothelial functions. If a plethora of shear stress-regulated molecular networks have been described in peripheral ECs, less is known about the molecular responses of microvascular brain ECs which constitute the blood-brain barrier (BBB). In this work, we investigated the response of human cerebral microvascular ECs to laminar physiological shear stress using the well characterized hCMEC/D3 cell line. Interestingly, we showed that hCMEC/D3 cells responded to shear stress by aligning perpendicularly to the flow direction, contrary to peripheral endothelial cells which aligned in the flow direction. Whole proteomic profiles were compared between hCMEC/D3 cells cultured either in static condition or under 5 or 10 dyn.cm-2 SS for 3 days. 3592 proteins were identified and expression levels were significantly affected for 3% of them upon both SS conditions. Pathway analyses were performed which revealed that most proteins overexpressed by SS refer to the antioxidant defense, probably mediated by activation of the NRF2 transcriptional factor. Regarding down-regulated proteins, most of them participate to the pro-inflammatory response, cell motility and proliferation. These findings confirm the induction of EC quiescence by laminar physiological SS and reveal a strong protective effect of SS on hCMEC/D3 cells, suggesting a similar effect on the BBB. Our results also showed that SS did not significantly increase expression levels nor did it affect the localization of junctional proteins and did not afect either the functional activity of several ABC transporters (P-glycoprotein and MRPs). This work provides new insights on the response of microvascular brain ECs to SS and on the importance of SS for optimizing in vitro BBB models.

The locus of Action of CGRPergic Monoclonal Antibodies Against Migraine: Peripheral Over Central Mechanisms

Migraine is a complex neurovascular disorder characterized by attacks of moderate to severe unilateral headache, accompanied by photophobia among other neurological signs. Although an arsenal of antimigraine agents is currently available in the market, not all patients respond to them. As Calcitonin Gene-Related Peptide (CGRP) plays a key role in the pathophysiology of migraine, CGRP receptor antagonists (gepants) have been developed. Unfortunately, further pharmaceutical development (for olcegepant and telcagepant) was interrupted due to pharmacokinetic issues observed during the Randomized Clinical Trials (RCT). On this basis, the use of monoclonal antibodies (mAbs; immunoglobulins) against CGRP or its receptor has recently emerged as a novel pharmacotherapy to treat migraines. RCT showed that these mAbs are effective against migraines producing fewer adverse events. Presently, the U.S. Food and Drug Administration approved four mAbs, namely: (i) erenumab; (ii) fremanezumab; (iii) galcanezumab; and (iv) eptinezumab. In general, specific antimigraine compounds exert their action in the trigeminovascular system, but the locus of action (peripheral vs. central) of the mAbs remains elusive. Since these mAbs have a molecular weight of ∼150 kDa, some studies rule out the relevance of their central actions as they seem unlikely to cross the Blood-Brain Barrier (BBB). Considering the therapeutic relevance of this new class of antimigraine compounds, the present review has attempted to summarize and discuss the current evidence on the probable sites of action of these mAbs.

Region-selective permeability of the blood-brain barrier to α-aminoisobutyric acid during thiamine deficiency and following its reversal

Thiamine deficiency (TD) results in focal lesions in several regions of the rat brain including the thalamus and inferior colliculus. Since alterations in blood-brain barrier (BBB) integrity may play a role in this damage, we have examined the influence of TD on the unidirectional blood-to-brain transfer constant (K_i) of the low molecular weight species α-aminoisobutyric acid (AIB) in vulnerable and non-vulnerable brain regions at different stages during progression of the disorder, and following its reversal with thiamine. Analysis of the regional distribution of K_i values showed early (day 10) increased transfer of [¹⁴C]-AIB across the BBB in the vulnerable medial thalamus as well as the non-vulnerable caudate and hippocampus. At the acute symptomatic stage (day 14), more widespread BBB permeability changes were detected in most areas including the lateral thalamus, inferior colliculus, and non-vulnerable cerebellum and pons. Twenty-four hours following thiamine replenishment, a heterogeneous pattern of increased BBB permeability was observed in which many structures maintained increased uptake of [¹⁴C]-AIB. No increase in the [³H]-dextran space, a marker of intravascular volume, was detected in brain regions during the progress of TD, suggesting that BBB permeability to this large tracer was unaffected. These results indicate that BBB opening i) occurs early during TD, ii) is not restricted to vulnerable areas of the brain, iii) is progressive, iv) persists for at least 24 h following treatment with thiamine, and v) is likely selective in nature, depending on the molecular species being transported.

Expression of angiopoietin-like 4 fibrinogen-like domain (cANGPTL4) increases risk of brain metastases in women with breast cancer

Background: Brain metastases challenge daily clinical practice, and the mechanisms by which cancer cells cross the blood-brain barrier remain largely undeciphered. Angiopoietin-like 4 (ANGPTL4) proteolytic fragments have controversial biological effects on endothelium permeability. Here, we studied the link between ANGPTL4 and the risk of brain metastasis in cancer patients.

Materials and Methods: From June 2015 to June 2016, serum samples from 113 cancer patients were prospectively collected, and ANGPTL4 concentrations were assessed. Using a murine model of brain metastases, we investigated the roles of nANGPTL4 and cANGPTL4, the two cleaved fragments of ANGPTL4, in the occurrence of brain metastases.

Results: An ANGPTL4 serum concentration over 0.1 ng/mL was associated with decreased overall-survival. Multivariate analyses found that only breast cancer brain metastases were significantly associated with elevated ANGPTL4 serum concentrations.

4T1 murine breast cancer cells were transfected with either nANGPTL4- or cANGPTL4-encoding cDNAs. Compared to mice injected with wild-type 4T1 cells, mice injected with nANGPTL4 cells had shorter median survival (p < 0.05), while mice injected with cANGPTL4 had longer survival (p < 0.01). On tissue sections, compared to wild-type mice, mice injected with nANGPTL4 cells had significantly larger surface areas of lung metastases (p < 0.01), and mice injected with cANGPTL4 had significantly larger surface areas of brain metastases (p < 0.01).

Conclusions: In this study, we showed that a higher expression of Angiopoietin-like 4 Fibrinogen-Like Domain (cANGPTL4) was associated with an increased risk of brain metastases in women with breast cancer.

Lipopolysaccharide Induced Opening of the Blood Brain Barrier on Aging 5XFAD Mouse Model

The development of neurotherapeutics for many neurodegenerative diseases has largely been hindered by limited pharmacologic penetration across the blood-brain barrier (BBB). Previous attempts to target and clear amyloid-β (Aβ) plaques, a key mediator of neurodegenerative changes in Alzheimer's disease (AD), have had limited clinical success due to low bioavailability in the brain because of the BBB. Here we test the effects of inducing an inflammatory response to disrupt the BBB in the 5XFAD transgenic mouse model of AD. Lipopolysaccharide (LPS), a bacterial endotoxin recognized by the innate immune system, was injected at varying doses. 24 hours later, mice were injected with either thioflavin S, a fluorescent Aβ-binding small molecule or 30 nm superparamagnetic iron oxide (SPIO) nanoparticles, both of which are unable to penetrate the BBB under normal physiologic conditions. Our results showed that when pretreated with 3.0 mg/kg LPS, thioflavin S can be found in the brain bound to Aβ plaques in aged 5XFAD transgenic mice. Following the same LPS pretreatment, SPIO nanoparticles could also be found in the brain. However, when done on wild type or young 5XFAD mice, limited SPIO was detected. Our results suggest that the BBB in aged 5XFAD mouse model is susceptible to increased permeability mediated by LPS, allowing for improved delivery of the small molecule thioflavin S to target Aβ plaques and SPIO nanoparticles, which are significantly larger than antibodies used in clinical trials for immunotherapy of AD. Although this approach demonstrated efficacy for improved delivery to the brain, LPS treatment resulted in significant weight loss even at low doses, resulting from the induced inflammatory response. These findings suggest inducing inflammation can improve delivery of small and large materials to the brain for improved therapeutic or diagnostic efficacy. However, this approach must be balanced with the risks of systemic inflammation.

Benchmarking in vitro tissue-engineered blood-brain barrier models

The blood–brain barrier (BBB) plays a key role in regulating transport into and out of the brain. With increasing interest in the role of the BBB in health and disease, there have been significant advances in the development of in vitro models. The value of these models to the research community is critically dependent on recapitulating characteristics of the BBB in humans or animal models. However, benchmarking in vitro models is surprisingly difficult since much of our knowledge of the structure and function of the BBB comes from in vitro studies. Here we describe a set of parameters that we consider a starting point for benchmarking and validation. These parameters are associated with structure (ultrastructure, wall shear stress, geometry), microenvironment (basement membrane and extracellular matrix), barrier function (transendothelial electrical resistance, permeability, efflux transport), cell function (expression of BBB markers, turnover), and co-culture with other cell types (astrocytes and pericytes). In suggesting benchmarks, we rely primarily on imaging or direct measurements in humans and animal models.

Detection of β-Amyloid by Sialic Acid Coated Bovine Serum Albumin Magnetic Nanoparticles in a Mouse Model of Alzheimer's Disease

The accumulation and formation of β-amyloid (Aβ) plaques in the brain are distinctive pathological hallmarks of Alzheimer's disease (AD). Designing nanoparticle (NP) contrast agents capable of binding with Aβ highly selectively can potentially facilitate early detection of AD. However, a significant obstacle is the blood brain barrier (BBB), which can preclude the entrance of NPs into the brain for Aβ binding. In this work, bovine serum albumin (BSA) coated NPs are decorated with sialic acid (NP-BSAx -Sia) to overcome the challenges in Aβ imaging in vivo. The NP-BSAx -Sia is biocompatible with high magnetic relaxivities, suggesting that they are suitable contrast agents for magnetic resonance imaging (MRI). The NP-BSAx -Sia binds with Aβ in a sialic acid dependent manner with high selectivities toward Aβ deposited on brains and cross the BBB in an in vitro model. The abilities of these NPs to detect Aβ in vivo in human AD transgenic mice by MRI are evaluated without the need to coinject mannitol to increase BBB permeability. T2 *-weighted MRI shows that Aβ plaques in mouse brains can be detected as aided by NP-BSAx -Sia, which is confirmed by histological analysis. Thus, NP-BSAx -Sia is a promising new tool for noninvasive in vivo detection of Aβ plaques.

Where are we? The anatomy of the murine cortical meninges revisited for intravital imaging, immunology, and clearance of waste from the brain

Rapid progress is being made in understanding the roles of the cerebral meninges in the maintenance of normal brain function, in immune surveillance, and as a site of disease. Most basic research on the meninges and the neural brain is now done on mice, major attractions being the availability of reporter mice with fluorescent cells, and of a huge range of antibodies useful for immunocytochemistry and the characterization of isolated cells. In addition, two-photon microscopy through the unperforated calvaria allows intravital imaging of the undisturbed meninges with sub-micron resolution. The anatomy of the dorsal meninges of the mouse (and, indeed, of all mammals) differs considerably from that shown in many published diagrams: over cortical convexities, the outer layer, the dura, is usually thicker than the inner layer, the leptomeninx, and both layers are richly vascularized and innervated, and communicate with the lymphatic system. A membrane barrier separates them and, in disease, inflammation can be localized to one layer or the other, so experimentalists must be able to identify the compartment they are studying. Here, we present current knowledge of the functional anatomy of the meninges, particularly as it appears in intravital imaging, and review their role as a gateway between the brain, blood, and lymphatics, drawing on information that is scattered among works on different pathologies.

Transport across the choroid plexus epithelium

The choroid plexus epithelium is a secretory epithelium par excellence. However, this is perhaps not the most prominent reason for the massive interest in this modest-sized tissue residing inside the brain ventricles. Most likely, the dominant reason for extensive studies of the choroid plexus is the identification of this epithelium as the source of the majority of intraventricular cerebrospinal fluid. This finding has direct relevance for studies of diseases and conditions with deranged central fluid volume or ionic balance. While the concept is supported by the vast majority of the literature, the implication of the choroid plexus in secretion of the cerebrospinal fluid was recently challenged once again. Three newer and promising areas of current choroid plexus-related investigations are as follows: 1) the choroid plexus epithelium as the source of mediators necessary for central nervous system development, 2) the choroid plexus as a route for microorganisms and immune cells into the central nervous system, and 3) the choroid plexus as a potential route for drug delivery into the central nervous system, bypassing the blood-brain barrier. Thus, the purpose of this review is to highlight current active areas of research in the choroid plexus physiology and a few matters of continuous controversy.

Crossing the Blood-Brain Barrier: Recent Advances in Drug Delivery to the Brain

CNS disorders are on the rise despite advancements in our understanding of their pathophysiological mechanisms. A major hurdle to the treatment of these disorders is the blood-brain barrier (BBB), which serves as an arduous janitor to protect the brain. Many drugs are being discovered for CNS disorders, which, however fail to enter the market because of their inability to cross the BBB. This is a pronounced challenge for the pharmaceutical fraternity. Hence, in addition to the discovery of novel entities and drug candidates, scientists are also developing new formulations of existing drugs for brain targeting. Several approaches have been investigated to allow therapeutics to cross the BBB. As the molecular structure of the BBB is better elucidated, several key approaches for brain targeting include physiological transport mechanisms such as adsorptive-mediated transcytosis, inhibition of active efflux pumps, receptor-mediated transport, cell-mediated endocytosis, and the use of peptide vectors. Drug-delivery approaches comprise delivery from microspheres, biodegradable wafers, and colloidal drug-carrier systems (e.g., liposomes, nanoparticles, nanogels, dendrimers, micelles, nanoemulsions, polymersomes, exosomes, and quantum dots). The current review discusses the latest advancements in these approaches, with a major focus on articles published in 2015 and 2016. In addition, we also cover the alternative delivery routes, such as intranasal and convection-enhanced diffusion methods, and disruption of the BBB for brain targeting.

Transferrin Receptor 1 Facilitates Poliovirus Permeation of Mouse Brain Capillary Endothelial Cells

As a possible route for invasion of the CNS, circulating poliovirus (PV) in the blood is believed to traverse the blood-brain barrier (BBB), resulting in paralytic poliomyelitis. However, the underlying mechanism is poorly understood. In this study, we demonstrated that mouse transferrin receptor 1 (mTfR1) is responsible for PV attachment to the cell surface, allowing invasion into the CNS via the BBB. PV interacts with the apical domain of mTfR1 on mouse brain capillary endothelial cells (MBEC4) in a dose-dependent manner via its capsid protein (VP1). We found that F-G, G-H, and H-I loops in VP1 are important for this binding. However, C-D, D-E, and E-F loops in VP1-fused Venus proteins efficiently penetrate MBEC4 cells. These results imply that the VP1 functional domain responsible for cell attachment is different from that involved in viral permeation of the brain capillary endothelium. We observed that co-treatment of MBEC4 cells with excess PV particles but not dextran resulted in blockage of transferrin transport into cells. Using the Transwell in vitro BBB model, transferrin co-treatment inhibited permeation of PV into MBEC4 cells and delayed further viral permeation via mTfR1 knockdown. With mTfR1 as a positive mediator of PV-host cell attachment and PV permeation of MBEC4 cells, our results indicate a novel role of TfR1 as a cellular receptor for human PV receptor/CD155-independent PV invasion of the CNS.

Review: in vitro microvessel models

A wide range of perfusable microvessel models have been developed, exploiting advances in microfabrication, microfluidics, biomaterials, stem cell technology, and tissue engineering. These models vary in complexity and physiological relevance, but provide a diverse tool kit for the study of vascular phenomena and methods to vascularize artificial organs. Here we review the state-of-the-art in perfusable microvessel models, summarizing the different fabrication methods and highlighting advantages and limitations.

Recent prospective of surface engineered Nanoparticles in the management of Neurodegenerative disorders

Clinically, the therapeutic outcomes in neurodegenerative disorders (NDs) by drug treatment are very limited, and the most insurmountable obstacle in the treatment of NDs is the blood-brain barrier (BBB), which provides the highest level of protection from xenobiotics. A great deal of attention still needs to be paid to overcome these barriers, and surface-engineered polymeric nanoparticles are emerging as innovative tools that are able to interact with the biological system at a molecular level for the desired response. The present review covers the potential importance of surface-structure-engineered nanoparticles to overcome the BBB for good bioavailability, and the evaluation of drug therapy in NDs.

Gadolinium-based contrast agents targeted to amyloid aggregates for the early diagnosis of Alzheimer's disease by MRI

While important efforts were made in the development of positron emission tomography (PET) tracers for the in vivo molecular diagnosis of Alzheimer's disease, very few investigations to develop magnetic resonance imaging (MRI) probes were performed. Here, a new generation of Gd(III)-based contrast agents (CAs) is proposed to detect the amyloid β-protein (Aβ) aggregates by MRI, one of the earliest biological hallmarks of the pathology. A building block strategy was used to synthesize a library of 16 CAs to investigate structure-activity relationships (SARs) on physicochemical properties and binding affinity for the Aβ aggregates. Three types of blocks were used to modulate the CA structures: (i) the Gd(III) chelates (Gd(III)-DOTA and Gd(III)-PCTA), (ii) the biovectors (2-arylbenzothiazole, 2-arylbenzoxazole and stilbene derivatives) and (iii) the linkers (neutrals, positives and negatives with several lengths). These investigations revealed unexpected SARs and a difficulty of these probes to cross the blood-brain barrier (BBB). General insights for the development of Gd(III)-based CAs to detect the Aβ aggregates are described.

Ferritin polarization and iron transport across monolayer epithelial barriers in mammals

Epithelial barriers are found in many tissues such as the intestine, kidney and brain where they separate the external environment from the body or a specific compartment from its periphery. Due to the tight junctions that connect epithelial barrier-cells (EBCs), the transport of compounds takes place nearly exclusively across the apical or basolateral membrane, the cell-body and the opposite membrane of the polarized EBC, and is regulated on numerous levels including barrier-specific adapted trafficking-machineries. Iron is an essential element but toxic at excess. Therefore, all iron-requiring organisms tightly regulate iron concentrations on systemic and cellular levels. In contrast to most cell types that control just their own iron homeostasis, EBCs also regulate homeostasis of the compartment they enclose or the body as a whole. Iron is transported across EBCs by specialized transporters such as the transferrin receptor and ferroportin. Recently, the iron storage protein ferritin was also attributed a role in the regulation of systemic iron homeostasis and we gathered evidence from the literature and original data that ferritin is polarized in EBC, suggesting also a role for ferritin in iron trafficking across EBCs.

Brain microvascular endothelial cells resist elongation due to curvature and shear stress

The highly specialized endothelial cells in brain capillaries are a key component of the blood-brain barrier, forming a network of tight junctions that almost completely block paracellular transport. In contrast to vascular endothelial cells in other organs, we show that brain microvascular endothelial cells resist elongation in response to curvature and shear stress. Since the tight junction network is defined by endothelial cell morphology, these results suggest that there may be an evolutionary advantage to resisting elongation by minimizing the total length of cell-cell junctions per unit length of vessel.

The blood-brain barrier: an engineering perspective

It has been more than 100 years since Paul Ehrlich reported that various water-soluble dyes injected into the circulation did not enter the brain. Since Ehrlich's first experiments, only a small number of molecules, such as alcohol and caffeine have been found to cross the blood-brain barrier, and this selective permeability remains the major roadblock to treatment of many central nervous system diseases. At the same time, many central nervous system diseases are associated with disruption of the blood-brain barrier that can lead to changes in permeability, modulation of immune cell transport, and trafficking of pathogens into the brain. Therefore, advances in our understanding of the structure and function of the blood-brain barrier are key to developing effective treatments for a wide range of central nervous system diseases. Over the past 10 years it has become recognized that the blood-brain barrier is a complex, dynamic system that involves biomechanical and biochemical signaling between the vascular system and the brain. Here we reconstruct the structure, function, and transport properties of the blood-brain barrier from an engineering perspective. New insight into the physics of the blood-brain barrier could ultimately lead to clinical advances in the treatment of central nervous system diseases.

Challenges in drug delivery to tumors of the central nervous system: an overview of pharmacological and surgical considerations

The majority of newly diagnosed brain tumors are treated with surgery, radiation, and the chemotherapeutic temozolomide. Development of additional therapeutics to improve treatment outcomes is complicated by the blood-brain barrier (BBB), which acts to protect healthy tissue from chemical insults. The high pressure found within brain tumors adds a challenge to local delivery of therapy by limiting the distribution of bolus injections. Here we discuss various drug delivery strategies, including convection-enhanced delivery, intranasal delivery, and intrathecal delivery, as well as pharmacological strategies for improving therapeutic efficacy, such as blood-brain barrier disruption.

Novel and emerging strategies in drug delivery for overcoming the blood-brain barrier

Two decades of molecular research have revealed the presence of transporters and receptors expressed in the brain vascular endothelium that provide potential novel targets for the rational design of blood-brain barrier-penetrating drugs. In this review, we briefly introduce the reader to the molecular characteristics of the blood-brain barrier that make this one of the most important obstacles towards the development of efficacious CNS drugs. We highlight recent attempts to rationally target influx and bidirectional transport systems expressed on the brain endothelial cell and avoid the important obstacle presented in the form of efflux transporters. Many of these approaches are highly innovative and show promise for future human application. Some of these approaches, however, have revealed significant limitations and are critiqued in this review. Nonetheless, these combined efforts have left the field of CNS drug delivery better positioned for developing novel approaches towards the rational design of CNS-penetrating drugs.

Minimally invasive molecular delivery into the brain using optical modulation of vascular permeability

Systemic delivery of bioactive molecules in the CNS is hampered by the blood-brain barrier, which has bottlenecked noninvasive physiological study of the brain and the development of CNS drugs. Here we report that irradiation with an ultrashort pulsed laser to the blood vessel wall induces transient leakage of blood plasma without compromising vascular integrity. By combining this method with a systemic injection, we delivered target molecules in various tissues, including the brain cortex. This tool allows minimally invasive local delivery of chemical probes, nanoparticles, and viral vectors into the brain cortex. Furthermore, we demonstrated astrocyte-mediated vasodilation in vivo without opening the skull, using this method to load a calcium indicator in conjunction with label-free photoactivation of astrocytes.

Physiologic upper limits of pore size of different blood capillary types and another perspective on the dual pore theory of microvascular permeability

BACKGROUND

Much of our current understanding of microvascular permeability is based on the findings of classic experimental studies of blood capillary permeability to various-sized lipid-insoluble endogenous and non-endogenous macromolecules. According to the classic small pore theory of microvascular permeability, which was formulated on the basis of the findings of studies on the transcapillary flow rates of various-sized systemically or regionally perfused endogenous macromolecules, transcapillary exchange across the capillary wall takes place through a single population of small pores that are approximately 6 nm in diameter; whereas, according to the dual pore theory of microvascular permeability, which was formulated on the basis of the findings of studies on the accumulation of various-sized systemically or regionally perfused non-endogenous macromolecules in the locoregional tissue lymphatic drainages, transcapillary exchange across the capillary wall also takes place through a separate population of large pores, or capillary leaks, that are between 24 and 60 nm in diameter. The classification of blood capillary types on the basis of differences in the physiologic upper limits of pore size to transvascular flow highlights the differences in the transcapillary exchange routes for the transvascular transport of endogenous and non-endogenous macromolecules across the capillary walls of different blood capillary types.

METHODS

The findings and published data of studies on capillary wall ultrastructure and capillary microvascular permeability to lipid-insoluble endogenous and non-endogenous molecules from the 1950s to date were reviewed. In this study, the blood capillary types in different tissues and organs were classified on the basis of the physiologic upper limits of pore size to the transvascular flow of lipid-insoluble molecules. Blood capillaries were classified as non-sinusoidal or sinusoidal on the basis of capillary wall basement membrane layer continuity or lack thereof. Non-sinusoidal blood capillaries were further sub-classified as non-fenestrated or fenestrated based on the absence or presence of endothelial cells with fenestrations. The sinusoidal blood capillaries of the liver, myeloid (red) bone marrow, and spleen were sub-classified as reticuloendothelial or non-reticuloendothelial based on the phago-endocytic capacity of the endothelial cells.

RESULTS

The physiologic upper limit of pore size for transvascular flow across capillary walls of non-sinusoidal non-fenestrated blood capillaries is less than 1 nm for those with interendothelial cell clefts lined with zona occludens junctions (i.e. brain and spinal cord), and approximately 5 nm for those with clefts lined with macula occludens junctions (i.e. skeletal muscle). The physiologic upper limit of pore size for transvascular flow across the capillary walls of non-sinusoidal fenestrated blood capillaries with diaphragmed fenestrae ranges between 6 and 12 nm (i.e. exocrine and endocrine glands); whereas, the physiologic upper limit of pore size for transvascular flow across the capillary walls of non-sinusoidal fenestrated capillaries with open 'non-diaphragmed' fenestrae is approximately 15 nm (kidney glomerulus). In the case of the sinusoidal reticuloendothelial blood capillaries of myeloid bone marrow, the transvascular transport of non-endogenous macromolecules larger than 5 nm into the bone marrow interstitial space takes place via reticuloendothelial cell-mediated phago-endocytosis and transvascular release, which is the case for systemic bone marrow imaging agents as large as 60 nm in diameter.

CONCLUSIONS

The physiologic upper limit of pore size in the capillary walls of most non-sinusoidal blood capillaries to the transcapillary passage of lipid-insoluble endogenous and non-endogenous macromolecules ranges between 5 and 12 nm. Therefore, macromolecules larger than the physiologic upper limits of pore size in the non-sinusoidal blood capillary types generally do not accumulate within the respective tissue interstitial spaces and their lymphatic drainages. In the case of reticuloendothelial sinusoidal blood capillaries of myeloid bone marrow, however, non-endogenous macromolecules as large as 60 nm in diameter can distribute into the bone marrow interstitial space via the phago-endocytic route, and then subsequently accumulate in the locoregional lymphatic drainages of tissues following absorption into the lymphatic drainage of periosteal fibrous tissues, which is the lymphatic drainage of myeloid bone marrow. When the ultrastructural basis for transcapillary exchange across the capillary walls of different capillary types is viewed in this light, it becomes evident that the physiologic evidence for the existence of aqueous large pores ranging between 24 and 60 nm in diameter in the capillary walls of blood capillaries, is circumstantial, at best.

Overcoming the challenges in the effective delivery of chemotherapies to CNS solid tumors

Locoregional therapies, such as surgery and intratumoral chemotherapy, do not effectively treat infiltrative primary CNS solid tumors and multifocal metastatic solid tumor disease of the CNS. It also remains a challenge to treat such CNS malignant solid tumor disease with systemic chemotherapies, although these lipid-soluble small-molecule drugs demonstrate potent cytotoxicity in vitro. Even in the setting of a 'normalized' tumor microenvironment, small-molecule drugs do not accumulate to effective concentrations in the vast majority of tumor cells, which is due to the fact that small-molecule drugs have short blood half-lives. It has been recently shown that drug-conjugated spherical lipid-insoluble nanoparticles within the 7-10 nm size range can deliver therapeutic concentrations of drug fraction directly into individual tumor cells following systemic administration, since these functionalized particles maintain peak blood concentrations for several hours and are smaller than the physiologic upper limit of pore size in the VEGF-derived blood capillaries of solid tumors, which is approximately 12 nm. In this article, the physiologic and ultrastructural basis of this novel translational approach for the treatment of CNS, as well as non-CNS, solid cancers is reviewed.

The physical chemistry of brain and neural cell membranes: an overview

The formation of cell membranes through the physical-chemical interaction of two hydrophilic colloidal fluids is applied to the formation of the membranes of brain and neural cells. Also described is the membrane mechanism of transfer of ions and compounds necessary for brain and neural cell functions into the cerebrospinal fluid through the blood-brain barrier. Changes in the cerebrospinal fluid giving rise to degradation of brain and neural cells and the formation of precipitates within the brain are considered. Monitoring of electrolyte changes in metabolic fluids is shown to be a possible method of predicting the onset of degenerate brain conditions.

Getting into the brain: approaches to enhance brain drug delivery

Being the most delicate organ of the body, the brain is protected against potentially toxic substances by the blood-brain barrier (BBB), which restricts the entry of most pharmaceuticals into the brain. The developmental process for new drugs for the treatment of CNS disorders has not kept pace with progress in molecular neurosciences because most of the new drugs discovered are unable to cross the BBB. The clinical failure of CNS drug delivery may be attributed largely to a lack of appropriate drug delivery systems. Localized and controlled delivery of drugs at their desired site of action is preferred because it reduces toxicity and increases treatment efficiency. The present review provides an insight into some of the recent advances made in the field of brain drug delivery.The various strategies that have been explored to increase drug delivery into the brain include (i) chemical delivery systems, such as lipid-mediated transport, the prodrug approach and the lock-in system; (ii) biological delivery systems, in which pharmaceuticals are re-engineered to cross the BBB via specific endogenous transporters localized within the brain capillary endothelium; (iii) disruption of the BBB, for example by modification of tight junctions, which causes a controlled and transient increase in the permeability of brain capillaries; (iv) the use of molecular Trojan horses, such as peptidomimetic monoclonal antibodies to transport large molecules (e.g. antibodies, recombinant proteins, nonviral gene medicines or RNA interference drugs) across the BBB; and (v) particulate drug carrier systems. Receptor-mediated transport systems exist for certain endogenous peptides, such as insulin and transferrin, enabling these molecules to cross the BBB in vivo.The use of polymers for local drug delivery has greatly expanded the spectrum of drugs available for the treatment of brain diseases, such as malignant tumours and Alzheimer's disease. In addition, various drug delivery systems (e.g. liposomes, microspheres, nanoparticles, nanogels and bionanocapsules) have been used to enhance drug delivery to the brain. Recently, microchips and biodegradable polymers have become important in brain tumour therapy.The intense search for alternative routes of drug delivery (e.g. intranasal drug delivery, convection-enhanced diffusion and intrathecal/intraventricular drug delivery systems) has been driven by the need to overcome the physiological barriers of the brain and to achieve high drug concentrations within the brain. For more than 30 years, considerable efforts have been made to enhance the delivery of therapeutic molecules across the vascular barriers of the CNS. The current challenge is to develop drug delivery strategies that will allow the passage of drug molecules through the BBB in a safe and effective manner.

Plasma lipoprotein beta-amyloid in subjects with Alzheimer's disease or mild cognitive impairment

BACKGROUND

Plasma amyloid beta-peptide (Abeta) can compromise the blood-brain barrier, contributing to cerebrovascular alterations and amyloid angiopathy in Alzheimer's disease (AD). The objectives of this study were to investigate the distribution of lipoprotein-bound plasma-Abeta isoforms.

METHODS

This involved a case-control study of subjects with AD or amnestic mild cognitive impairment (MCI) versus controls. Lipoprotein Abeta distribution was determined in fasted plasma. For assessment of chylomicron homeostasis in the postabsorptive state, subjects were bled 4 h after a low-fat meal. The main outcome measures were plasma lipoprotein Abeta isoform distribution and lipid homeostasis.

RESULTS

We found the majority of plasma Abeta to be associated with triglyceride-rich lipoproteins (TRLs) encompassing chylomicrons, VLDL and IDL. For all lipoprotein groups, Abeta1-40 was the predominant isoform, accounting for approximately 50% of the total. Thereafter, equivalent amounts of the isoforms 1-42, 2-40, 1-38, 1-37 and 1-39 were found. Abeta1-37, Abeta1-38 and Abeta2-40 isoforms were significantly enriched within the TRL fraction of AD/MCI subjects and similar trends were observed for isoforms Abeta1-39, Abeta1-40 and Abeta1-42. Lipoprotein-Abeta was inversely associated with plasma total- and LDL cholesterol. AD/MCI subjects were not dyslipidaemic, however, there was evidence of accumulation of chylomicrons in the postabsorptive state.

CONCLUSIONS

Our data show that Abeta was found to be associated with plasma lipoproteins, especially those enriched with triglyceride. We find that Abeta may be increased in normolipidaemic AD subjects, commensurate with possible disturbances in postprandial lipoprotein homeostasis.

Blood-brain barrier studies with special reference to epileptic seizures

A review is given of the structure and function of the blood-brain barrier (BBB) with special reference to the situation during and after epileptic seizures including electroconvulsive therapy (ECT). Based on the literature describing different models of epilepsy the development of the theories concerning the early observations of a BBB dysfunction in connection with seizure activity is given, and gross morphology, ultrastructural and dynamic findings are briefly reviewed.

Molecular determinants of endothelial transcytosis and their role in endothelial permeability

Caveolae transcytosis with its diverse mechanisms-fluid phase, adsorptive, and receptor-mediated-plays an important role in the continuous exchange of molecules across the endothelium. We will discuss key features of endothelial transcytosis and caveolae that have been studied recently and have increased our understanding of caveolae function in transcytosis at the molecular level. During transcytosis, caveolae "pinch off" from the plasma membrane to form discrete vesicular carriers that shuttle to the opposite front of endothelial cells, fuse with the plasma membrane, and discharge their cargo into the perivascular space. Endothelial transcytosis exhibits distinct properties, the most important being rapid and efficient coupling of endocytosis to exocytosis on opposite plasma membrane. We address herein the membrane fusion-fission reactions that underlie transcytosis. Caveolae move across the endothelial cells with their cargo predominantly in the fluid phase through an active process that bypasses the lysosomes. Endothelial transcytosis is a constitutive process of vesicular transport. Recent studies show that transcytosis can be upregulated in response to pathological stimuli. Transcytosis via caveolae is an important route for the regulation of endothelial barrier function and may participate in different vascular diseases.

Neuropeptides as possible targets in sleep disorders

Insomnia and hypersomnia are frequent sleep disorders, and they are most often treated pharmacologically with hypnotics and wake-promoting compounds. These compounds act on classical neurotransmitter systems, such as benzodiazepines on GABA-A receptors, and amfetamine-like stimulants on monoaminergic terminals to modulate neurotransmission. In addition, acetylcholine, amino acids, lipids and proteins (cytokines) and peptides, are known to significantly modulate sleep and are, therefore, possibly involved in the pathophysiology of some sleep disorders. Due to the recent developments of molecular biological techniques, many neuropeptides have been newly identified, and some are found to significantly modulate sleep. It was also discovered that the impairment of the hypocretin/orexin neurotransmission (a recently isolated hypothalamic neuropeptide system) is the major pathophysiology of narcolepsy, and hypocretin replacement therapy is anticipated to treat the disease in humans. In this article, the authors briefly review the history of neuropeptide research, followed by the sleep modulatory effects of various neuropeptides. Finally, general strategies for the pharmacological therapeutics targeting the peptidergic systems for sleep disorders are discussed.

Methods to assess drug permeability across the blood-brain barrier

Much research has focussed on the development of novel therapeutic agents to target various central nervous system disorders, however less attention has been given to determining the potential of such agents to permeate the blood-brain barrier (BBB), a factor that will ultimately govern the effectiveness of these agents in man. In order to assess the potential for novel compounds to permeate the BBB, various in-vitro, in-vivo and in-silico methods may be employed. Although in-vitro models (such as primary cell culture and immortalized cell lines) are useful as a screening method and can appropriately rank compounds in order of BBB permeability, they often correlate poorly to in-vivo brain uptake due to down-regulation of some BBB-specific transporters. In-vivo models (such as the internal carotid artery single injection or perfusion, intravenous bolus injection, brain efflux index and intracerebral microdialysis) provide more accurate information regarding brain uptake, and these can be complemented with novel imaging techniques (such as magnetic resonance imaging and positron emission tomography), although such methods are not suited to high-throughput permeability assessment. This paper reviews current methods used for assessing BBB permeability and highlights the particular advantages and disadvantages associated with each method, with a particular focus on methods suitable for moderate- to high-throughput screening.

Molecular biology of the blood-brain barrier

Molecular biological investigations into the brain capillary endothelium and microvasculature, which forms the blood-brain barrier (BBB) in vivo, can provide the platform for the discovery and the molecular cloning of BBB-specific genes. Novel BBB genes can be discovered with either a genomics-based approach such as subtractive suppressive hybridization, or a proteomics approach using subtractive antibody expression cloning. BBB-specific genes are disproportionately transporter genes encoding either for carrier-mediated transporters, active efflux transporters, or receptor-mediated transporters. The discovery of new BBB transporters can lead to the development of new approaches to brain drug delivery using endogenous brain endothelial transporters.

Molecular modeling of blood–brain barrier nutrient transporters: In silico basis for evaluation of potential drug delivery to the central nervous system

For drugs that act in the brain, the blood-brain barrier (BBB) is a considerable physical barrier which influences the distribution of drugs to the brain. The BBB is essentially impermeable for hydrophilic and/or charged compounds. Nutrient membrane transporters have an important physiological role in the transport of essential substances across the BBB required for normal brain function. We and others have shown that these transporters may have utility as drug delivery vectors, thereby increasing brain distribution of these compounds via these systems. In this review, we evaluate molecular (in silico) models of BBB transport proteins. Few BBB membrane transporters have been crystallized, but their crystal structures have a possibility for use in homology modeling. Other techniques commonly used are 2D quantitative structure-activity relationships (QSAR), as well as 3D-QSAR techniques including comparative molecular field analysis (CoMFA) and comparative similarity index analysis (CoMSIA). Each of these models provides valuable information for ascertaining their potential basis for BBB transport and brain drug delivery.

Chlorpyrifos Alters Functional Integrity and Structure of an In Vitro BBB Model: Co-cultures of Bovine Endothelial Cells and Neonatal Rat Astrocytes

The blood-brain barrier (BBB) is a structural and functional interface between the circulatory system and the brain. Organophosphorous compounds such as chlorpyrifos (CPF) may cross the BBB and disrupt BBB integrity and function. To determine events that may contribute to CPF toxicity, we used an in vitro BBB model in which bovine microvascular endothelial cells (BMEC) and neonatal rat astrocytes were co-cultured. We hypothesized that CPF is metabolized by the BBB leading to an inhibition of esterase activity and a disruption of the BBB. The co-culturing of BMECs and astrocytes resulted in tight junction formation as determined by electron microscopy, electrical resistance and western blot analysis of two tight junction-associated proteins (ZO-1 and e-cadherin). We observed time dependent increases in ZO-1 and e-cadherin expression and electrical resistance during BBB formation, which were maximal after 9-13 days of co-culturing. The CPF concentration and production of its metabolites were monitored by HPLC following 24 h exposure to CPF on the luminal side of the BBB. We found that the BBB metabolized CPF, with the metabolite 2,3,6-trichloro-2-pyridinol being the major product. CPF and its metabolites were detected on the abluminal side of the BBB suggesting that CPF crossed this barrier. CPF was also detected intracellularly and on the membrane inserts. At tested concentrations (0.1-10 microM), CPF inhibited both carboxylesterase (CaE) and cholinesterase (ChE) activities in BMECs by 43-100%, while CPF-oxon totally inhibited CaE and ChE activity in concentrations as low as 0.1 microM. CPF also caused a concentration-dependent decrease in electrical resistance, with significant inhibition observed at 1 nM and complete loss at 1 microM. These data show that low concentrations of CPF and its metabolites are present within the BBB. CPF and its metabolites, especially CPF-oxon, contribute to the inhibition of CaE and ChE activity, as well as the alteration of BBB integrity and structure.

Drug delivery to Brain by Microparticulate Systems

The site specific delivery of chemotherapeutic agents allows maximum concentration of an agent at a desired body site. This area specific drug delivery decreases the unwanted systemic distribution and decreases toxicity of the administered drugs. Blood-Brain Barrier (BBB) is considered to be an obstacle in delivering large number of drugs to brain. The endothelial cells forming the tubular capillaries in the brain are cemented together by intercellular tight junctions. In this way, the BBB has an important role in providing a stable extracellular environment in the central nervous system. Lack of fenestrations, very few pinocytotic vesicles, and more mitochondria are other differences of the brain capillaries which play important role in transport of drugs to brain (Fig.1). The purpose of this paper is to summarise the methods for BBB permeability modifications and to focus on various examples in delivering drugs, especially neuroncology and neuroactive drugs, to brain by microparticulate systems.

Brain uptake of thiamine-coated nanoparticles

Recently, a novel nanoparticle (NP) comprised of emulsifying wax and Brij 78 was shown to have significant brain uptake using the in-situ rat brain perfusion technique. To further these studies and to specifically target brain, we have incorporated thiamine as a surface ligand on the nanoparticles. Solid nanoparticles were prepared from oil-in-water microemulsion precursors. Nanoparticles were radiolabeled and a thiamine ligand (thiamine linked to distearoylphosphatidylethanolamine via a polyethylene glycol spacer) was coated on the surface of the nanoparticles. Initial experiments focused on assessing uptake of [3H]nanoparticles with and without thiamine surface ligands. Biodistribution nanoparticle studies were also carried out in BALB/c mice. The results showed: (1) the effectiveness of using microemulsions as precursors to engineer nanoparticles, (2) kinetic modeling for brain uptake of nanoparticles with and without the thiamine surface ligands, and (3) initial data suggesting mechanisms for nanoparticle brain entry. Comparison of NP brain uptake demonstrated that the thiamine-coated nanoparticle associated with the blood-brain barrier (BBB) thiamine transporter and had an increased K(in) between 45 and 120 s (thiamine coated NP 9.8 +/- 1.1 x 10(-3) ml/s/g versus uncoated NPs; 7.0 +/- 0.3 x 10(-3) ml/s/g). It was concluded that the thiamine ligand facilitated binding and/or association with blood-brain barrier thiamine transporters, which may be a viable mechanism for nanoparticle mediated brain drug delivery.