Transferrin receptor on endothelium of brain capillaries

Qualitative and semiquantitative analysis of the protein coronas associated to different functionalized nanoparticles

AIM

The investigation on protein coronas (PCs) adsorbed onto nanoparticle (NP) surface is representing an open issue due to difficulties in detection and clear isolation of the adsorbed proteins. In this study, we investigated protocols able to isolate the compositions of PCs of three polymeric NPs.

MATERIALS & METHODS

Unfunctionalized NPs and two functionalized NPs were considered as proof-of-concept for the qualitative and semiquantitative analysis of both the corona levels (stably or weakly adsorbed coronas [SC/WC]) of these different nanocarriers.

RESULTS

The protocols applied were able to discriminate between the SC and WC. In particular, experimental results indicated that stably adsorbed coronas are prevalently composed by ApoE, while WC by albumin in all the NPs. Otherwise, some differences in WC could be correlated with surface functionalization.

CONCLUSION

This experimental approach allows characterizing the whole PCs, proposing a protocol for isolation of different types of proteins composing PCs.

Peptide Mediated Brain Delivery of Nano- and Submicroparticles: A Synergistic Approach

The brain is a complex, regulated organ with a highly controlled access mechanism: The Blood-Brain Barrier (BBB). The selectivity of this barrier is a double-edged sword, being both its greatest strength and weakness. This weakness is evident when trying to target therapeutics against diseases within the brain. Diseases such as metastatic brain cancer have extremely poor prognosis due to the poor permeability of many therapeutics across the BBB. Peptides can be designed to target BBB receptors and gain access to the brain by transcytosis. These peptides (known as BBB-shuttles) can carry compounds, usually excluded from the brain, across the BBB. BBB-shuttles are limited by poor loading of therapeutics and degradation of the peptide and cargo. Likewise, nano- submicro- and microparticles can be fine-tuned to limit their degradation and with high loading of therapeutics. However, most nano- and microparticles' core materials completely lack efficient targeting, with a few selected materials able to cross the BBB passively. Combining the selectivity of peptides with the high loading potential of nano-, microparticles offers an exciting strategy to develop novel, targeted therapeutics towards many brain disorders and diseases. Nevertheless, at present the field is diverse, in both scope and nomenclature, often with competing or contradictory names. In this review, we will try to address some of these issues and evaluate the current state of peptide mediated nano,-microparticle transport to the brain, analyzing delivery vehicle type and peptide design, the two key components that must act synergistically for optimal therapeutic impact.

Preparation and Characterization of Biocompatible Chitosan Nanoparticles for Targeted Brain Delivery of Peptides

Here, we describe a nanocarrier system that can transfer chitosan nanoparticles loaded with either small peptides such as the caspase inhibitor Z-DEVD-FMK or a large peptide like basic fibroblast growth factor across the blood-brain barrier. The nanoparticles are selectively directed to the brain and are not measurably taken up by the liver and spleen. Intravital fluorescent microscopy provides an opportunity to study the penetration kinetics of nanoparticles loaded with fluorescent agents such as Nile red. Nanoparticles functionalized with anti-transferrin antibody and loaded with peptides efficiently provided neuroprotection when systemically administered either as a formulation bearing a single peptide or a mixture of them. Failure of brain permeation of the nanoparticles after inhibition of vesicular transcytosis by imatinib as well as when nanoparticles were not functionalized with anti-transferrin antibody indicates that this nanomedicine formulation is rapidly transported across the blood-brain barrier by receptor-mediated transcytosis.

Isolation of blood-brain barrier-crossing antibodies from a phage display library by competitive elution and their ability to penetrate the central nervous system

The blood-brain barrier (BBB) is a formidable obstacle for delivery of biologic therapeutics to central nervous system (CNS) targets. Whilst the BBB prevents passage of the vast majority of molecules, it also selectively transports a wide variety of molecules required to maintain brain homeostasis. Receptor-mediated transcytosis is one example of a macromolecule transport system that is employed by cells of the BBB to supply essential proteins to the brain and which can be utilized to deliver biologic payloads, such as antibodies, across the BBB. In this study, we performed phage display selections on the mouse brain endothelial cell line, bEND.3, to enrich for antibody single-chain variable fragments (scFvs) that could compete for binding with a known BBB-crossing antibody fragment, FC5. A number of these scFvs were converted to IgGs and characterized for their ability to bind to mouse, rat and human brain endothelial cells, and subsequent ability to transport across the BBB. We demonstrated that these newly identified BBB-targeting IgGs had increased brain exposure when delivered peripherally in mice and were also able to transport a biologically active molecule, interleukin-1 receptor antagonist (IL-1RA), into the CNS. The antagonism of the interleukin-1 system within the CNS can result in the relief of neuropathic pain. We demonstrated that the BBB-targeting IgGs were able to elicit an analgesic response in a mouse model of nerve ligation-induced hypersensitivity when fused to IL-1RA.

Potential Pathways for CNS Drug Delivery Across the Blood-Cerebrospinal Fluid Barrier

The blood-brain interfaces restrict the cerebral bioavailability of pharmacological compounds. Various drug delivery strategies have been developed to improve drug penetration into the brain. Most strategies target the microvascular endothelium forming the blood-brain barrier proper. Targeting the blood-cerebrospinal fluid (CSF) barrier formed by the epithelium of the choroid plexuses in addition to the blood-brain barrier may offer added-value for the treatment of central nervous system diseases. For instance, targeting the CSF spaces, adjacent tissue, or the choroid plexuses themselves is of interest for the treatment of neuroinflammatory and infectious diseases, cerebral amyloid angiopathy, selected brain tumors, hydrocephalus or neurohumoral dysregulation. Selected CSF-borne materials seem to reach deep cerebral structures by mechanisms that need to be understood in the context of chronic CSF delivery. Drug delivery through both barriers can reduce CSF sink action towards parenchymal drugs. Finally, targeting the choroid plexus-CSF system can be especially relevant in the context of neonatal and pediatric diseases of the central nervous system. Transcytosis appears the most promising mechanism to target in order to improve drug delivery through brain barriers. The choroid plexus epithelium displays strong vesicular trafficking and secretory activities that deserve to be explored in the context of cerebral drug delivery. Folate transport and exosome release into the CSF, plasma protein transport, and various receptor-mediated endocytosis pathways may prove useful mechanisms to exploit for efficient drug delivery into the CSF. This calls for a clear evaluation of transcytosis mechanisms at the blood-CSF barrier, and a thorough evaluation of CSF drug delivery rates.

Co-coating of receptor-targeted drug nanocarriers with anti-phagocytic moieties enhances specific tissue uptake versus non-specific phagocytic clearance

Nanocarriers (NCs) help improve the performance of therapeutics, but their removal by phagocytes in the liver, spleen, tissues, etc. diminishes this potential. Although NC functionalization with polyethylene glycol (PEG) lowers interaction with phagocytes, it also reduces interactions with tissue cells. Coating NCs with CD47, a protein expressed by body cells to avoid phagocytic removal, offers an alternative. Previous studies showed that coating CD47 on non-targeted NCs reduces phagocytosis, but whether this alters binding and endocytosis of actively-targeted NCs remains unknown. To evaluate this, we used polymer NCs targeted to ICAM-1, a receptor overexpressed in many diseases. Co-coating of CD47 on anti-ICAM NCs reduced macrophage phagocytosis by ∼50% for up to 24 h, while increasing endothelial-cell targeting by ∼87% over control anti-ICAM/IgG NCs. Anti-ICAM/CD47 NCs were endocytosed via the CAM-mediated pathway with efficiency similar (0.99-fold) to anti-ICAM/IgG NCs. Comparable outcomes were observed for NCs targeted to PECAM-1 or transferrin receptor, suggesting broad applicability. When injected in mice, anti-ICAM/CD47 NCs reduced liver and spleen uptake by ∼30-50% and increased lung targeting by ∼2-fold (∼10-fold over IgG NCs). Therefore, co-coating NCs with CD47 and targeting moieties reduces macrophage phagocytosis and improves targeted uptake. This strategy may significantly improve the efficacy of targeted drug NCs.

Selection and identification of transferrin receptor-specific peptides as recognition probes for cancer cells

Since the transferrin receptor (CD71 or TFRC) is known to be highly expressed in numerous cancers, CD71 has become an attractive target in cancer research. Acquiring specific molecular probes for CD71, such as small molecular ligands, aptamers, peptides, or antibodies, is of great importance for cancer cell recognition and capture. In this work, we chose CD71 as the target for phage display, and after four rounds of positive selection and one round of negative selection, the specific phage library was enriched. After verification and sequence analysis, six peptides were identified to be able to bind to CD71 with high specificity. The specific recognition of the CD71-positive cells was confirmed by flow cytometry and confocal microscopy. Competition experiments demonstrated that peptide Y1 and transferrin (TF) were bound to distinct sites on CD71, indicating that peptide Y1 could replace TF as a potential probe for cell imaging and drug delivery, thus avoiding competition by endogenous TF and side effects. Graphical abstract Six peptides were successfully isolated using in vitro biopanning against CD71 with high specificity and affinity. Peptides Y1 and Y2 would be powerful tools in biosensors and biomedicine due to their unique properties.

Targeting transferrin receptors at the blood-brain barrier improves the uptake of immunoliposomes and subsequent cargo transport into the brain parenchyma

Drug delivery to the brain is hampered by the presence of the blood-brain barrier, which excludes most molecules from freely diffusing into the brain, and tightly regulates the active transport mechanisms that ensure sufficient delivery of nutrients to the brain parenchyma. Harnessing the possibility of delivering neuroactive drugs by way of receptors already present on the brain endothelium has been of interest for many years. The transferrin receptor is of special interest since its expression is limited to the endothelium of the brain as opposed to peripheral endothelium. Here, we investigate the possibility of delivering immunoliposomes and their encapsulated cargo to the brain via targeting of the transferrin receptor. We find that transferrin receptor-targeting increases the association between the immunoliposomes and primary endothelial cells in vitro, but that this does not correlate with increased cargo transcytosis. Furthermore, we show that the transferrin receptor-targeted immunoliposomes accumulate along the microvessels of the brains of rats, but find no evidence for transcytosis of the immunoliposome. Conversely, the increased accumulation correlated both with increased cargo uptake in the brain endothelium and subsequent cargo transport into the brain. These findings suggest that transferrin receptor-targeting is a relevant strategy of increasing drug exposure to the brain.

A polymeric temozolomide nanocomposite against orthotopic glioblastoma xenograft: tumor-specific homing directed by nestin

The development of effective therapeutic strategies for glioblastoma faces challenges such as modulating the blood brain barrier (BBB) for drug influx and selectively targeting tumor cells. Nanocarrier drug delivery strategies are functionalized to enhance vascular permeability. We engineered superparamagnetic iron oxide nanoparticle (SPION) based polymeric nanocomposites (84.37 ± 12.37 nm / 101.56 ± 7.42 nm) embedding temozolomide (TMZ) targeted against glioblastoma by tagging an antibody against nestin, a stem cell marker, and transferrin / polysorbate-80 to permeate the BBB. The targeting and therapeutic efficacy of the nanocomposite resulted in enhanced permeability across the BBB in an orthotopic glioblastoma xenograft model. Sustained release of TMZ from the nanocomposite contributed to enhanced tumor cell death while sparing normal brain cells as evidenced through micro SPECT/CT analysis. The functionalized nanocomposites showed significant reductions in tumor volume compared to pure TMZ, as substantiated by reduced proliferation markers such as proliferating cell nuclear antigen (PCNA) and Ki-67. We report here a novel targeted TMZ delivery strategy using a potent homing moiety, nestin, tagged to a polymeric nanocomposite to target glioblastoma. In addition to tumor targeting, this study constitutes a broad horizon for enhanced therapeutic efficacy with further scope for capitalizing on the magnetic properties of SPION for targeted killing of cancer cells while sparing normal tissues.

Development of antibody-modified chitosan nanoparticles for the targeted delivery of siRNA across the blood-brain barrier as a strategy for inhibiting HIV replication in astrocytes

RNA interference (RNAi)-mediated gene silencing offers a novel treatment and prevention strategy for human immunodeficiency virus (HIV) infection. HIV was found to infect and replicate in human brain cells and can cause neuroinfections and neurological deterioration. We designed dual-antibody-modified chitosan/small interfering RNA (siRNA) nanoparticles to deliver siRNA across the blood-brain barrier (BBB) targeting HIV-infected brain astrocytes as a strategy for inhibiting HIV replication. We hypothesized that transferrin antibody and bradykinin B2 antibody could specifically bind to the transferrin receptor (TfR) and bradykinin B2 receptor (B2R), respectively, and deliver siRNA across the BBB into astrocytes as potential targeting ligands. In this study, chitosan nanoparticles (CS-NPs) were prepared by a complex coacervation method in the presence of siRNA, and antibody was chemically conjugated to the nanoparticles. The antibody-modified chitosan nanoparticles (Ab-CS-NPs) were spherical in shape, with an average particle size of 235.7 ± 10.2 nm and a zeta potential of 22.88 ± 1.78 mV. The therapeutic potential of the nanoparticles was evaluated based on their cellular uptake and gene silencing efficiency. Cellular accumulation and gene silencing efficiency of Ab-CS-NPs in astrocytes were significantly improved compared to non-modified CS-NPs and single-antibody-modified CS-NPs. These results suggest that the combination of anti-Tf antibody and anti-B2 antibody significantly increased the knockdown effect of siRNA-loaded nanoparticles. Thus, antibody-mediated dual-targeting nanoparticles are an efficient and promising delivery strategy for inhibiting HIV replication in astrocytes. Graphical abstract Graphic representation of dual-antibody-conjugated chitosan nanoparticles for the targeted delivery of siRNA across the blood-brain barrier (BBB) for inhibiting HIV replication in astrocytes. a Nanoparticle delivery to the BBB and penetration. b TfR-mediated transcytosis of nanoparticles across the epithelial cells. c B2R-mediated endocytosis of nanoparticles in astrocytes. d The molecular interactions between HIV-1 Tat protein and Cyclin T1 and Tip110 cellular proteins. e A schematic representation of chitosan nanoparticles with its components. RNAPII RNA polymerase II, TAR transactivation response RNA element, LTR long terminal repeat, Ab antibody, CS chitosan, TPP tripolyphosphate.

Dual-functional drug liposomes in treatment of resistant cancers

Efficacy of regular chemotherapy is significantly hampered by multidrug resistance (MDR) and severe systemic toxicity. The reduced toxicity has been evidenced after administration of drug liposomes, consisting of the first generation of regular drug liposomes, the second generation of long-circulation drug liposomes, and the third generation of targeting drug liposomes. However, MDR of cancers remains as an unsolved issue. The objective of this article is to review the dual-functional drug liposomes, which demonstrate the potential in overcoming MDR. Herein, dual-functional drug liposomes are referring to the drug-containing phospholipid bilayer vesicles that possess a dual-function of providing the basic efficacy of drug and the extended effect of the drug carrier. They exhibit unique roles in treatment of resistant cancer via circumventing drug efflux caused by adenosine triphosphate binding cassette (ABC) transporters, eliminating cancer stem cells, destroying mitochondria, initiating apoptosis, regulating autophagy, destroying supply channels, utilizing microenvironment, and silencing genes of the resistant cancer. As the prospect of an estimation, dual-functional drug liposomes would exhibit more strength in their extended function, hence deserving further investigation for clinical validation.

Recent advancements in liposomes targeting strategies to cross blood-brain barrier (BBB) for the treatment of Alzheimer's disease

In this modern era, with the help of various advanced technologies, medical science has overcome most of the health-related issues successfully. Though, some diseases still remain unresolved due to various physiological barriers. One such condition is Alzheimer; a neurodegenerative disorder characterized by progressive memory impairment, behavioral abnormalities, mood swing and disturbed routine activities of the person suffering from. It is well known to all that the brain is entirely covered by a protective layer commonly known as blood brain barrier (BBB) which is responsible to maintain the homeostasis of brain by restricting the entry of toxic substances, drug molecules, various proteins and peptides, small hydrophilic molecules, large lipophilic substances and so many other peripheral components to protect the brain from any harmful stimuli. This functionally essential structure creates a major hurdle for delivery of any drug into the brain. Still, there are some provisions on BBB which facilitate the entry of useful substances in the brain via specific mechanisms like passive diffusion, receptor-mediated transcytosis, carrier-mediated transcytosis etc. Another important factor for drug transport is the selection of a suitable drug delivery systems like, liposome, which is a novel drug carrier system offering a potential approach to resolving this problem. Its unique phospholipid bilayer structure (similar to physiological membrane) had made it more compatible with the lipoidal layer of BBB and helps the drug to enter the brain. The present review work focused on various surface modifications with functional ligand (like lactoferrin, transferrin etc.) and carrier molecules (such as glutathione, glucose etc.) on the liposomal structure to enhance its brain targeting ability towards the successful treatment of Alzheimer disease.

Serum Hepcidin Levels, Iron Dyshomeostasis and Cognitive Loss in Alzheimer's Disease

This pilot study examined the status of the master iron regulatory peptide, hepcidin, and peripheral related iron parameters in Alzheimer's disease (AD) and mild cognitive impairment patients, and evaluated the relationship between iron dyshomeostasis and amyloid-beta (Aβ), cognitive assessment tests, neuroimaging and clinical data. Frozen serum samples from the Oregon Tissue Bank were used to measure serum levels of hepcidin, ferritin, Aβ40, Aβ42 using enzyme-linked immunosorbent assay. Serum transferrin levels were determined indirectly as total iron binding capacity, serum iron was measured and the percent saturation of transferrin calculated. The study variables were correlated with the patients' existing cognitive assessment tests, neuroimaging, and clinical data. Hepcidin, and iron-related proteins tended to be higher in AD patients than controls, reaching statistical significance for ferritin, whereas Aβ40, Aβ42 serum levels tended to be lower. Patients with pure AD had three times higher serum hepcidin levels than controls; gender differences in hepcidin and iron-related proteins were observed. Patient stratification based on clinical dementia rating-sum of boxes revealed significantly higher levels of iron and iron-related proteins in AD patients in the upper 50% as compared to controls, suggesting that iron dyshomeostasis worsens as cognitive impairment increases. Unlike Aβ peptides, iron and iron-related proteins showed significant association with cognitive assessment tests, neuroimaging, and clinical data. Hepcidin and iron-related proteins comprise a group of serum biomarkers that relate to AD diagnosis and AD disease progression. Future studies should determine whether strategies targeted to diminishing hepcidin synthesis/secretion and improving iron homeostasis could have a beneficial impact on AD progression.

A dual-mediated liposomal drug delivery system targeting the brain: rational construction, integrity evaluation across the blood-brain barrier, and the transporting mechanism to glioma cells

As the global population ages, cancer rates increase worldwide, and degenerative diseases of the central nervous system (CNS), brain tumors, and inflammation threaten human health more frequently. We designed a dual-mediated (receptor-mediated and adsorption-mediated) liposome, named transferrin–cell penetrating peptide–sterically stabilized liposome (TF-CPP-SSL), to improve therapy for gliomas through combining molecular recognition of transferrin receptors (TF-Rs) on the blood–brain barrier (BBB) and glioma cells with the internalization and lysosomal escaping ability of CPP. Based on the systematic investigation of structure–activity relations on the cellular level, we constructed TF-CPP-SSL rationally by conjugating TF and CPP moieties to the liposomes via PEG_3.4K and PEG_2.0K, respectively, and found the optimum densities of TF and CPP were 1.8% and 4%, respectively. These liposomes had the highest targeting efficacy for brain microvascular endothelial cell and C6 cell uptake but avoided capture by normal cells. Fluorescence resonance energy transfer technology and coculture models of BBB and glioma C6 cells indicated that TF-CPP-SSL was transported across the BBB without drug leakage, liposome breakup, or cleavage of ligand. TF-CPP-SSL offered advantages for crossing the BBB and entering into glioma C6 cells. Real-time confocal viewing revealed that TF-CPP-SSL was entrapped in endosomes of glioma C6 cells and then escaped from lysosomes successfully to release the liposomal contents into the cytosol. Entrapped contents, such as doxorubicin, could then enter the nucleus to exert pharmacological effects.

Nanoformulated Antiretrovirals for Penetration of the Central Nervous System: State of the Art

The central nervous system is a very challenging HIV-1 sanctuary. But, despite complete suppression of plasmatic viral replication with current antiretroviral therapy, signs of HIV-1 replication can still be found in the cerebrospinal fluid in some patients. The main limitation to achieving HIV-1 eradication from the brain is related to the suboptimal concentrations of antiretrovirals within this site, due to their low permeation across the blood-brain barrier. In recent years, a number of reliable nanotechnological strategies have been developed with the aim of enhancing antiretroviral drug penetration across the blood-brain barrier. The aim of this review is to provide an overview of the different nanoformulated antiretrovirals, used in both clinical and preclinical studies, that are designed to improve their delivery into the brain by active or passive permeation mechanisms through the barrier. Different nanotechnological approaches have proven successful for optimizing antiretrovirals delivery to the central nervous system, with a likely benefit for HIV-associated neurocognitive disorders and a more debated contribution to the complete eradication of the HIV-1 infection.

Non-invasive approaches for drug delivery to the brain based on the receptor mediated transport

The blood brain barrier (BBB) is a physical and biochemical barrier that prevents entry of toxic compounds into brain for preserving homeostasis. However, the BBB also strictly limits influx of most therapeutic agents into the brain. One promising method for overcoming this problem to deliver drugs is receptor mediated transport (RMT) system, which employs the vesicular trafficking machinery to transport substrates across the BBB endothelium in a noninvasive manner. The conjugates of drug or drug-loaded vector linked with appropriate ligands specifically binds to the endogenous targeting receptor on the surface of the endothelial cells. Then drugs could enter the cell body by means of transcytosis and eventual releasing into the brain parenchyma. Over the past 20years, there have been significant developments of RMT targeting strategies. Here, we will review the recent advance of various promising RMT systems and discuss the capability of these approaches for drug delivery to the brain.

Multifunctional Dendrimers for Drug Nanocarriers

Dendrimers are nanosized, monodisperse, highly branched polymers with well-defined topological structure which have attracted much attention for drug delivery recently. To further improve the performance of dendrimers in drug delivery, various functional dendrimers are developed by decorating the dendrimers with targeting agents, imaging agents, or stimuli-sensitive moieties. They show good biocompatibility, visibility, tumor targeting and stimuli-sensitive properties for drug or gene delivery. This chapter will focus on the design of multifunctional nanocarriers based on the dendrimers. Therefore, the chapter will provide the ideas for designing the dendrimers based nanocarriers for controllable drug delivery and let more people know the development of dendrimers for drug delivery in recent years.

Protein engineering approaches for regulating blood-brain barrier transcytosis

The blood brain barrier (BBB) presents a challenge for the delivery of brain therapeutics. Trans-BBB delivery methods that use targeting vectors to coopt the vesicle trafficking machinery of BBB endothelial cells have been developed, but these are often hampered by limited flux through the BBB. A solution to this problem lies in the semi-rational engineering of BBB targeting vectors. Leveraging knowledge of intracellular trafficking, researchers have begun to tune selected binding properties of the vector-receptor interaction. Engineered binding affinity, avidity and pH-sensitivity have been shown to affect binding, intracellular sorting and release, ultimately leading to increased brain uptake of the targeting vector and its associated cargo. However, each targeted receptor may exhibit differential responses to engineered binding properties, illustrating the need to better understand vector-receptor interactions and trafficking dynamics.

Exploiting transferrin receptor for delivering drugs across the blood-brain barrier

Delivery of large molecule drugs across the blood brain barrier is increasingly being seen as an achievable goal. Several technologies have been described where following peripheral administration the molecules can be detected in the brain. Foremost amongst these technologies are antibodies against the transferrin receptor. Following a burst of publications in the very early twenty first century, excitement seemed to wane as contrary data started to emerge. Over the last few years antibodies against transferrin receptor have again started to raise hopes of successful drug delivery to the central nervous system, as protein engineering techniques have allowed a more detailed understanding of the antibody properties necessary for successful transport across the blood brain barrier.

Transferrin receptor expression and role in transendothelial transport of transferrin in cultured brain endothelial monolayers

Receptor-mediated transcytosis of the transferrin receptor has been suggested as a potential transport system to deliver therapeutic molecules into the brain. Recent studies have however shown that therapeutic antibodies, which have been reported to cross the brain endothelium, reach greater brain exposure when the affinity of the antibodies to the transferrin receptor is lowered. The lower affinity of the antibodies to the transferrin receptor facilitates the dissociation from the receptor within the endosomal compartments, which may indicate that the receptor itself does not necessarily move across the endothelial cells by transcytosis. The aim of the present study was to investigate transferrin receptor expression and role in transendothelial transferrin transport in cultured bovine brain endothelial cell monolayers. Transferrin receptor mRNA and protein levels were investigated in endothelial mono-cultures and co-cultures with astrocytes, as well as in freshly isolated brain capillaries using qPCR, immunocytochemistry and Western blotting. Transendothelial transport and luminal association of holo-transferrin was investigated using [¹²⁵I]holo-transferrin or [⁵⁹Fe]-transferrin. Transferrin receptor mRNA expression in all cell culture configurations was lower than in freshly isolated capillaries, but the expression slightly increased during six days of culture. The mRNA expression levels were similar in mono-cultures and co-cultures. Immunostaining demonstrated comparable transferrin receptor localization patterns in mono-cultures and co-cultures. The endothelial cells demonstrated an up-regulation of transferrin receptor mRNA after treatment with the iron chelator deferoxamine. The association of [¹²⁵I]holo-transferrin and [⁵⁹Fe]-transferrin to the endothelial cells was inhibited by an excess of unlabeled holo-transferrin, indicating receptor mediated association. However, over time the cell associated [⁵⁹Fe]-label exceeded that of [¹²⁵I]holo-transferrin, which could indicate release of iron in the endothelial cells and receptor recycling. Luminal-to-abluminal transport of [¹²⁵I]holo-transferrin across endothelial cell monolayers was low and not inhibited by unlabeled holo-transferrin. This indicated that transendothelial transferrin transport was independent of transferrin receptor-mediated transcytosis.

In Vitro Permeation of FITC-loaded Ferritins Across a Rat Blood-brain Barrier: a Model to Study the Delivery of Nanoformulated Molecules

Brain microvascular endothelial cells, supported by pericytes and astrocytes endfeet, are responsible for the low permeation of large hydrosoluble drugs through the blood-brain barrier (BBB), causing difficulties for effective pharmacological therapies. In recent years, different strategies for promoting brain targeting have aimed to improve drug delivery and activity at this site, including innovative nanosystems for drug delivery across the BBB. In this context, an in vitro approach based on a simplified cellular model of the BBB provides a useful tool to investigate the effect of nanoformulations on the trans-BBB permeation of molecules. This study describes the development of a double-layer BBB, consisting of co-cultured commercially available primary rat brain microvascular endothelial cells and astrocytes. A multiparametric approach for the validation of the model, based on the measurement of the transendothelial electrical resistance and the apparent permeability of a high molecular weight dextran, is also described. As proof of concept for the employment of this BBB model to study the effect of different nanoformulations on the translocation of fluorescent molecules across the barrier, we describe the use of fluorescein isothiocyanate (FITC), loaded into ferritin nanoparticles. The ability of ferritins to improve the trans-BBB permeation of FITC was demonstrated by flux measurements and confocal microscopy analyses. The results suggest this is a useful system for validating nanosystems for delivery of drugs across the BBB.

Immunotherapy in Parkinson's Disease: Micromanaging Alpha-Synuclein Aggregation

Currently, several α-synuclein immunotherapies are being tested in experimental Parkinson’s disease models and in clinical trials. Recent research has revealed that α-synuclein is not just an intracellular synaptic protein but also exists extracellularly. Moreover, the transfer of misfolded α-synuclein between cells might be a crucial step in the process leading to a progressive increase in deposition of α-synuclein aggregates throughout the Parkinson’s disease brain. The revelation that α-synuclein is present outside cells has increased the interest in antibody-based therapies and opens up for the notion that microglia might play a key role in retarding Parkinson’s disease progression. The objectives of this review are to describe and contrast the use of active and passive immunotherapy in treating α-synucleinopathies and highlight the likely important role of microglia in clearing misfolded α-synuclein from the extracellular space.

Aβ-Immunotherapeutic strategies: a wide range of approaches for Alzheimer's disease treatment

Current therapies to treat Alzheimer's disease (AD) are focused on ameliorating symptoms instead of treating the underlying causes of AD. The accumulation of amyloid β (Aβ) oligomers, whether by an increase in production or by a decrease in clearance, has been described as the seed that initiates the pathological cascade in AD. Developing therapies to target these species is a vital step in improving AD treatment. Aβ-immunotherapy, especially passive immunotherapy, is a promising approach to reduce the Aβ burden. Up to now, several monoclonal antibodies (mAbs) have been tested in clinical trials on humans, but none of them have passed Phase III. In all likelihood, these trials failed mainly because patients with mild-to-moderate AD were recruited, and thus treatment may have been too late to be effective. Therefore, many ongoing clinical trials are being conducted in patients at the prodromal stage. New structures based on antibody fragments have been engineered intending to improve efficacy and safety. This review presents the properties of this variety of developing treatments and provides a perspective on state-of-the-art of passive Aβ-immunotherapy in AD.

Iron in the Brain: An Important Contributor in Normal and Diseased States

Iron is essential for normal neurological function because of its role in oxidative metabolism and because it is a cofactor in the synthesis of neurotransmitters and myelin. In the past several years, there has been increased attention to the importance of oxidative stress in the central nervous system. Iron is the most important inducer of reactive oxygen species, therefore, the relation of iron to neurodegenerative processes is more appreciated today than it was a few years ago. Nevertheless, despite this increased attention and awareness, our knowledge of iron metabolism in the brain at the cellular and molecular levels is still limited. Iron is distributed in a heterogeneous fashion among the different regions and cells of the brain. This regional and cellular heterogeneity is preserved across many species. Brain iron concentrations are not static; they increase with age and in many diseases and decrease when iron is deficient in the diet. In infants and children, insufficient iron in the diet is associated with decreased brain iron and with changes in behavior and cognitive functioning. Abnormal iron accumulation in the diseased brain areas and, in some cases, alterations in iron-related proteins have been reported in many neurodegenerative diseases, including Hallervorden-Spatz syndrome, Alzheimer’s disease, Parkinson’s disease, and Friedreich’s ataxia. There is strong evidence for iron-mediated oxidative damage as a primary contributor to cell death in these disorders. Demyelinating diseases, such as multiple sclerosis, especially warrant study in relation to iron availability. Myelin synthesis and maintenance have a high iron requirement, thus, oligodendrocytes must have a relatively high and constant supply of iron. However, the high oxygen utilization, high density of lipids, and high iron content of white matter all combine to increase the risk of oxidative damage. We review here the current knowledge of the normal metabolism of iron in the brain and the suspected role of iron in neuropathology.

Convection-Enhanced and Local Delivery of Targeted Cytotoxins in the Treatment of Malignant Gliomas

Despite advances in our knowledge about the genesis, molecular biology, and natural history of malignant gliomas and the use of a multi-disciplinary approach to their treatment, patients harboring this diagnosis continue to face a grim prognosis. At the time of diagnosis, patients typically undergo surgery for the establishment of a histologic diagnosis, the reduction of tumor burden, and the relief of mass effect, with the maintenance of the patient's neurological function in mind. This is followed by the administration of adjuvant therapeutics, including radiation therapy and chemotherapy.

Many investigational agents with laboratory evidence of efficacy against malignant gliomas have not met their promise in the clinical setting, largely due to the barriers that they must overcome to reach the tumor at a therapeutically meaningful concentration for a durable period of time. The relevant aspects of the blood-brain barrier, blood-tumor barrier, and blood-cerebrospinal fluid barrier, as they pertain to the delivery of agents to the tumor, will be discussed along with the strategies devised to circumvent them. This discussion will be followed by a description of agents currently in preclinical and clinical development, many of which are the result of intense ongoing research into the molecular biology of gliomas.

In vitro models of the blood-brain barrier: An overview of commonly used brain endothelial cell culture models and guidelines for their use

The endothelial cells lining the brain capillaries separate the blood from the brain parenchyma. The endothelial monolayer of the brain capillaries serves both as a crucial interface for exchange of nutrients, gases, and metabolites between blood and brain, and as a barrier for neurotoxic components of plasma and xenobiotics. This "blood-brain barrier" function is a major hindrance for drug uptake into the brain parenchyma. Cell culture models, based on either primary cells or immortalized brain endothelial cell lines, have been developed, in order to facilitate in vitro studies of drug transport to the brain and studies of endothelial cell biology and pathophysiology. In this review, we aim to give an overview of established in vitro blood-brain barrier models with a focus on their validation regarding a set of well-established blood-brain barrier characteristics. As an ideal cell culture model of the blood-brain barrier is yet to be developed, we also aim to give an overview of the advantages and drawbacks of the different models described.

Transferrin Receptor Controls AMPA Receptor Trafficking Efficiency and Synaptic Plasticity

Transferrin receptor (TFR) is an important iron transporter regulating iron homeostasis and has long been used as a marker for clathrin mediated endocytosis. However, little is known about its additional function other than iron transport in the development of central nervous system (CNS). Here we demonstrate that TFR functions as a regulator to control AMPA receptor trafficking efficiency and synaptic plasticity. The conditional knockout (KO) of TFR in neural progenitor cells causes mice to develop progressive epileptic seizure, and dramatically reduces basal synaptic transmission and long-term potentiation (LTP). We further demonstrate that TFR KO remarkably reduces the binding efficiency of GluR2 to AP2 and subsequently decreases AMPA receptor endocytosis and recycling. Thus, our study reveals that TFR functions as a novel regulator to control AMPA trafficking efficiency and synaptic plasticity.

Apicobasal polarity of brain endothelial cells

Normal brain homeostasis depends on the integrity of the blood-brain barrier that controls the access of nutrients, humoral factors, and immune cells to the CNS. The blood-brain barrier is composed mainly of brain endothelial cells. Forming the interface between two compartments, they are highly polarized. Apical/luminal and basolateral/abluminal membranes differ in their lipid and (glyco-)protein composition, allowing brain endothelial cells to secrete or transport soluble factors in a polarized manner and to maintain blood flow. Here, we summarize the basic concepts of apicobasal cell polarity in brain endothelial cells. To address potential molecular mechanisms underlying apicobasal polarity in brain endothelial cells, we draw on investigations in epithelial cells and discuss how polarity may go awry in neurological diseases.

Mathematical PKPD and safety model of bispecific TfR/BACE1 antibodies for the optimization of antibody uptake in brain

Treatment of diseases of the central nervous system by monoclonal antibodies may be limited by the restricted uptake of antibodies across the blood-brain barrier (BBB). An antibody targeting transferrin receptor (TfR) has been shown to take advantage of the receptor-mediated transcytosis properties of TfR in order to cross the BBB in mice, with the uptake in the brain being dependent on the affinity to TfR. In the bispecific format with arms targeting both TfR and β-secretase 1 (BACE1), altering the affinity to TfR has been shown to impact systemic exposure and safety profiles. In this work, a mathematical model incorporating pharmacokinetic/pharmacodynamic (PKPD) and safety profiles is developed for bispecific TfR/BACE1 antibodies with a range of affinities to TfR in order to guide candidate selection. The model captures the dependence of both systemic and brain exposure on TfR affinity and the subsequent impact on brain Aβ40 lowering and circulating reticulocyte levels. Model simulations identify the optimal affinity for the TfR arm of the bispecific to maximize Aβ reduction while maintaining reticulocyte levels. The model serves as a useful tool to prioritize and optimize preclinical studies and has been used to support the selection of additional candidates for further development.

The blood-brain barrier

In autoimmune neurologic disorders, the blood-brain barrier (BBB) plays a central role in immunopathogenesis, since this vascular interface is an entry path for cells and effector molecules of the peripheral immune system to reach the target organ, the central nervous system (CNS). The BBB's unique anatomic structure and the tightly regulated interplay of its cellular and acellular components allow for maintenance of brain homeostasis, regulation of influx and efflux, and protection from harm; these ensure an optimal environment for the neuronal network to function properly. In both health and disease, the BBB acts as mediator between the periphery and the CNS. For example, immune cell trafficking through the cerebral vasculature is essential to clear microbes or cell debris from neural tissues, while poorly regulated cellular transmigration can underlie or worsen CNS pathology. In this chapter, we focus on the specialized multicellular structure and function of the BBB/neurovascular unit and discuss how BBB breakdown can precede or be a consequence of neuroinflammation. We introduce the blood-cerebrospinal fluid barrier and include a brief aside about evolutionary aspects of barrier formation and refinements. Lastly, since restoration of barrier function is considered key to ameliorate neurologic disease, we speculate about new therapeutic avenues to repair a damaged BBB.

Plasmid pORF-hTRAIL targeting to glioma using transferrin-modified polyamidoamine dendrimer

A gene drug delivery system for glioma therapy based on transferrin (Tf)-modified polyamidoamine dendrimer (PAMAM) was prepared. Gene drug, tumor necrosis factor-related apoptosis-inducing ligand (hTRAIL)-encoding plasmid open reading frame (pORF-hTRAIL, Trail), was condensed by Tf-modified PAMAM to form nanoparticles (NPs). PAMAM-PEG-Tf/DNA NPs showed higher cellular uptake, in vitro gene expression, and cytotoxicity than PAMAM-PEG/DNA NPs in C6 cells. The in vivo targeting efficacy of NPs was visualized by ex vivo fluorescence imaging. Tf-modified NPs showed obvious glioma-targeting trend. Plasmid encoding green fluorescence protein (GFP) was also condensed by modified or unmodified PAMAM to evaluate the in vivo gene expression level. The PAMAM-PEG-Tf/plasmid encoding enhanced green fluorescence protein (pEGFP) NPs exhibited higher GFP expression level than PAMAM-PEG/pEGFP NPs. TUNEL assay revealed that Tf-modified NPs could induce much more tumor apoptosis. The median survival time of PAMAM-PEG-Tf/Trail-treated rats (28.5 days) was longer than that of rats treated with PAMAM-PEG/Trail (25.5 days), temozolomide (24.5 days), PAMAM-PEG-Tf/pEGFP (19 days), or saline (17 days). The therapeutic effect was further confirmed by magnetic resonance imaging. This study demonstrated that targeting gene delivery system had potential application for the treatment of glioma.

Targeting specific cells in the brain with nanomedicines for CNS therapies

Treatment of Central Nervous System (CNS) disorders still remains a major clinical challenge. The Blood-Brain Barrier (BBB), known as the major hindrance, greatly limits therapeutics penetration into the brain. Moreover, even though some therapeutics can cross BBB based on their intrinsic properties or via the use of proper nanoscale delivery vehicles, their therapeutic efficacy is still often limited without the specific uptake of drugs by the cancer or disease-associated cells. As more studies have started to elucidate the pathological roles of major cells in the CNS (for example, microglia, neurons, and astrocytes) for different disorders, nanomedicines that can enable targeting of specific cells in these diseases may provide great potential to boost efficacy. In this review, we aim to briefly cover the pathological roles of endothelial cells, microglia, tumor-associated microglia/macrophage, neurons, astrocytes, and glioma in CNS disorders and to highlight the recent advances in nanomedicines that can target specific disease-associated cells. Furthermore, we summarized some strategies employed in nanomedicine to achieve specific cell targeting or to enhance the drug neuroprotective effects in the CNS. The specific targeting at the cellular level by nanotherapy can be a more precise and effective means not only to enhance the drug availability but also to reduce side effects.

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.

Liposomes Coloaded with Elacridar and Tariquidar To Modulate the P-Glycoprotein at the Blood-Brain Barrier

This study prepared three liposomal formulations coloaded with elacridar and tariquidar to overcome the P-glycoprotein-mediated efflux at the blood-brain barrier. Their pharmacokinetics, brain distribution, and impact on the model P-glycoprotein substrate, loperamide, were compared to those for the coadministration of free elacridar plus free tariquidar. After intravenous administration in rats, elacridar and tariquidar in conventional liposomes were rapidly cleared from the bloodstream. Their low levels in the brain did not improve the loperamide brain distribution. Although elacridar and tariquidar in PEGylated liposomes exhibited 2.6 and 1.9 longer half-lives than free elacridar and free tariquidar, respectively, neither their Kp for the brain nor the loperamide brain distribution was improved. However, the conjugation of OX26 F(ab')2 fragments to PEGylated liposomes increased the Kps for the brain of elacridar and tariquidar by 1.4- and 2.1-fold, respectively, in comparison to both free P-gp modulators. Consequently, the Kp for the brain of loperamide increased by 2.7-fold. Moreover, the plasma pharmacokinetic parameters and liver distribution of loperamide were not modified by the PEGylated OX26 F(ab')2 immunoliposomes. Thus, this formulation represents a promising tool for modulating the P-glycoprotein-mediated efflux at the blood-brain barrier and could improve the brain uptake of any P-glycoprotein substrate that is intended to treat central nervous system diseases.

A convenient method for hTfR1 inclusion body purification

Human transferrin receptor, referred as hTfR1, is ubiquitously expressed at low levels in most normal human tissues; however, the expression level of hTfR1 at the blood-brain barrier (BBB) and in tumor tissues is relatively higher. hTfR1 is a type II homodimeric transmembrane protein. The extracellular domain of hTfR1 consists of three domains: helical domain, apical, and protease-like domain. In order to prepare hTfR1 antibody, which can be utilized to deliver drugs across BBB through receptor-mediated endocytosis, we began to express the nonligand binding domain of hTfR1 in Escherichia coli BL21 Transetta (DE3). The TfR1 gene was first obtained from HepG2 cells by reverse-transcription polymerase chain reaction (RT-PCR) and then inserted into pET 32a(c+) vector. The protein was expressed in the form of inclusion body with extremely high purity by the E. coli BL21 Transetta (DE3), and the purity was further improved by size-exclusion chromatography. The Western blot test indicated that the recombinant protein was TfR1 as expected. Above all, this report provided a convenient protocol that could be fulfilled in order to prepare hTfR1 inclusion body, which failed to be purified by an Ni(2+) affinity column.

Deficiency of Calcium-Independent Phospholipase A2 Beta Induces Brain Iron Accumulation through Upregulation of Divalent Metal Transporter 1

Mutations in PLA2G6 have been proposed to be the cause of neurodegeneration with brain iron accumulation type 2. The present study aimed to clarify the mechanism underlying brain iron accumulation during the deficiency of calcium-independent phospholipase A2 beta (iPLA2β), which is encoded by the PLA2G6 gene. Perl's staining with diaminobenzidine enhancement was used to visualize brain iron accumulation. Western blotting was used to investigate the expression of molecules involved in iron homeostasis, including divalent metal transporter 1 (DMT1) and iron regulatory proteins (IRP1 and 2), in the brains of iPLA2β-knockout (KO) mice as well as in PLA2G6-knockdown (KD) SH-SY5Y human neuroblastoma cells. Furthermore, mitochondrial functions such as ATP production were examined. We have discovered for the first time that marked iron deposition was observed in the brains of iPLA2β-KO mice since the early clinical stages. DMT1 and IRP2 were markedly upregulated in all examined brain regions of aged iPLA2β-KO mice compared to age-matched wild-type control mice. Moreover, peroxidized lipids were increased in the brains of iPLA2β-KO mice. DMT1 and IRPs were significantly upregulated in PLA2G6-KD cells compared with cells treated with negative control siRNA. Degeneration of the mitochondrial inner membrane and decrease of ATP production were observed in PLA2G6-KD cells. These results suggest that the genetic ablation of iPLA2β increased iron uptake in the brain through the activation of IRP2 and upregulation of DMT1, which may be associated with mitochondrial dysfunction.