A Human Blood‐Arachnoid Barrier Atlas of Transporters, Receptors, Enzymes, Tight Junction and Marker Proteins: Comparison with Dog and Pig in Absolute Abundance

The canine blood-brain barrier in health and disease: focus on brain protection

This review examines the role of the canine blood-brain barrier (BBB) in health and disease, focusing on the impact of the multidrug resistance (MDR) transporter P-glycoprotein (P-gp) encoded by the ABCB1/MDR1 gene. The BBB is critical in maintaining central nervous system homeostasis and brain protection against xenobiotics and environmental drugs that may be circulating in the blood stream. We revise key anatomical, histological and functional aspects of the canine BBB and examine the role of the ABCB1/MDR1 gene mutation in specific dog breeds that exhibit reduced P-gp activity and disrupted drug brain pharmacokinetics. The review also covers factors that may disrupt the canine BBB, including the actions of aging, canine cognitive dysfunction, epilepsy, inflammation, infection, traumatic brain injury, among others. We highlight the critical importance of this barrier in maintaining central nervous system homeostasis and protecting against xenobiotics and conclude that a number of neurological-related diseases may increase vulnerability of the BBB in the canine species and discuss its profound impacts on canine health.

Assessing central nervous system drug delivery

INTRODUCTION

Delivering drugs to the central nervous system (CNS) remains a major challenge due to the blood-brain barrier, restricting the entry of drugs into the brain. This limitation contributes to the ongoing lack of effective treatments for CNS diseases. To improve the process of drug discovery and development, it is crucial to streamline methods that measure clinically relevant parameters, allowing for good selection of drug candidates.

AREA COVERED

In this paper, we discuss the essential prerequisites for successful CNS drug delivery and review relevant methods. We emphasize the need for closer collaboration between in vitro and in vivo scientists to improve the relevance of these methods and increase the success rate of developing effective CNS therapies. While our focus is on small molecule drugs, we also touch on some aspects of larger molecules.

EXPERT OPINION

Significant progress has been made in recent years in method development and their application. However, there is still work to be done before the use of in silico models, in vitro cell systems, and AI can consistently offer meaningful correlations and relationships to clinical data. This gap is partly due to limited patient data, but a lot can be achieved through in vivo research in animal models.

Breaking boundaries: role of the brain barriers in metastatic process

Brain metastases (BMs) are the most common intracranial tumors in adults and occur 3-10 times more frequently than primary brain tumors. Despite intensive multimodal therapies, including resection, radiotherapy, and chemotherapy, BMs are associated with poor prognosis and remain challenging to treat. BMs predominantly originate from primary lung (20-56%), breast (5-20%), and melanoma (7-16%) tumors, although they can arise from other cancer types less frequently. The metastatic cascade is a multistep process involving local invasion, intravasation into the bloodstream or lymphatic system, extravasation into normal tissue, and colonization of the distal site. After reaching the brain, circulating tumor cells (CTCs) breach the blood-brain barrier (BBB).The selective permeability of the BBB poses a significant challenge for therapeutic compounds, limiting the treatment efficacy of BMs. Understanding the mechanisms of tumor cell interactions with the BBB is crucial for the development of effective treatments. This review provides an in-depth analysis of the brain barriers, including the BBB, blood-spinal cord barrier, blood-meningeal barrier, blood-arachnoid barrier, and blood-cerebrospinal fluid barrier. It explores the molecular and cellular components of these barriers and their roles in brain metastasis, highlighting the importance of this knowledge for identifying druggable targets to prevent or limit BM formation.

Reimagining the meninges from a neuroimmune perspective: a boundary, but not peripheral

Recent advances in neuroscience have transformed our understanding of the meninges, the layers surrounding the central nervous system (CNS). Two key findings have advanced our understanding: researchers identified cranial bone marrow as a reservoir for meningeal immune cells, and rediscovered a brain lymphatic system. Once viewed merely as a protective barrier, the meninges are now recognized as a dynamic interface crucial for neuroimmune interactions. This shift in perspective highlights their unique role in maintaining CNS balance, shaping brain development, and regulating responses to injury and disease. This review synthesizes the latest insights into meningeal anatomy and function, with a focus on newly identified structures such as dural-associated lymphoid tissues (DALT) and arachnoid cuff exit (ACE) points. We also examine the diverse immune cell populations within the meninges and their interactions with the CNS, underscoring the emerging view of the meninges as active participants in brain immunity. Finally, we outline critical unanswered questions about meningeal immunity, proposing directions for future research. By addressing these knowledge gaps, we aim to deepen our understanding of the meninges' role in brain health and disease, potentially paving the way for novel therapeutic approaches.

Advances and controversies in meningeal biology

The dura, arachnoid and pia mater, as the constituent layers of the meninges, along with cerebrospinal fluid in the subarachnoid space and ventricles, are essential protectors of the brain and spinal cord. Complemented by immune cells, blood vessels, lymphatic vessels and nerves, these connective tissue layers have held many secrets that have only recently begun to be revealed. Each meningeal layer is now known to have molecularly distinct types of fibroblasts. Cerebrospinal fluid clearance through peripheral lymphatics and lymph nodes is well documented, but its routes and flow dynamics are debated. Advances made in meningeal immune functions are also debated. This Review considers the cellular and molecular structure and function of the dura, arachnoid and pia mater in the context of conventional views, recent progress, and what is uncertain or unknown. The hallmarks of meningeal pathophysiology are identified toward developing a more complete understanding of the meninges in health and disease.

An overview: Radiotracers and nano-radiopharmaceuticals for diagnosis of Parkinson's disease

Parkinson's disease (PD) is a widespread progressive neurodegenerative disease. Clinical diagnosis approaches are insufficient to provide an early and accurate diagnosis before a substantial of loss of dopaminergic neurons. PET and SPECT can be used for accurate and early diagnosis of PD by using target-specific radiotracers. Additionally, the importance of BBB penetrating targeted nanosystems has increased in recent years. This article reviews targeted radiopharmaceuticals used in clinics and novel nanocarriers for research purposes of PD imaging.

Blood-brain barrier transporters: a translational consideration for CNS delivery of neurotherapeutics

INTRODUCTION

Successful neuropharmacology requires optimization of CNS drug delivery and, by extension, free drug concentrations at brain molecular targets. Detailed assessment of blood-brain barrier (BBB) physiological characteristics is necessary to achieve this goal. The 'next frontier' in CNS drug delivery is targeting BBB uptake transporters, an approach that requires evaluation of brain endothelial cell transport processes so that effective drug accumulation and improved therapeutic efficacy can occur.

AREAS COVERED

BBB permeability of drugs is governed by tight junction protein complexes (i.e., physical barrier) and transporters/enzymes (i.e., biochemical barrier). For most therapeutics, a component of blood-to-brain transport involves passive transcellular diffusion. Small molecule drugs that do not possess acceptable physicochemical characteristics for passive permeability may utilize putative membrane transporters for CNS uptake. While both uptake and efflux transport mechanisms are expressed at the brain microvascular endothelium, uptake transporters can be targeted for optimization of brain drug delivery and improved treatment of neurological disease states.

EXPERT OPINION

Uptake transporters represent a unique opportunity to optimize brain drug delivery by leveraging the endogenous biology of the BBB. A rigorous understanding of these transporters is required to improve translation from the bench to clinical trials and stimulate the development of new treatment paradigms for neurological diseases.

Proton-Coupled Oligopeptide Transport (Slc15) in the Brain: Past and Future Research

This mini-review describes the role of the solute carrier (SLC)15 family of proton-coupled oligopeptide transporters (POTs) and particularly Pept2 (Slc15A2) and PhT1 (Slc15A4) in the brain. That family transports endogenous di- and tripeptides and peptidomimetics but also a number of drugs. The review focuses on the pioneering work of David E. Smith in the field in identifying the impact of PepT2 at the choroid plexus (the blood-CSF barrier) as well as PepT2 and PhT1 in brain parenchymal cells. It also discusses recent findings and future directions in relation to brain POTs including cellular and subcellular localization, regulatory pathways, transporter structure, species differences and disease states.

Organic cation transporter 2 contributes to SSRI antidepressant efficacy by controlling tryptophan availability in the brain

Selective serotonin reuptake inhibitors (SSRI) are common first-line treatments for major depression. However, a significant number of depressed patients do not respond adequately to these pharmacological treatments. In the present preclinical study, we demonstrate that organic cation transporter 2 (OCT2), an atypical monoamine transporter, contributes to the effects of SSRI by regulating the routing of the essential amino acid tryptophan to the brain. Contrarily to wild-type mice, OCT2-invalidated mice failed to respond to prolonged fluoxetine treatment in a chronic depression model induced by corticosterone exposure recapitulating core symptoms of depression, i.e., anhedonia, social withdrawal, anxiety, and memory impairment. After corticosterone and fluoxetine treatment, the levels of tryptophan and its metabolites serotonin and kynurenine were decreased in the brain of OCT2 mutant mice compared to wild-type mice and reciprocally tryptophan and kynurenine levels were increased in mutants' plasma. OCT2 was detected by immunofluorescence in several structures at the blood-cerebrospinal fluid (CSF) or brain-CSF interface. Tryptophan supplementation during fluoxetine treatment increased brain concentrations of tryptophan and, more discreetly, of 5-HT in wild-type and OCT2 mutant mice. Importantly, tryptophan supplementation improved the sensitivity to fluoxetine treatment of OCT2 mutant mice, impacting chiefly anhedonia and short-term memory. Western blot analysis showed that glycogen synthase kinase-3β (GSK3β) and mammalian/mechanistic target of rapamycin (mTOR) intracellular signaling was impaired in OCT2 mutant mice brain after corticosterone and fluoxetine treatment and, conversely, tryptophan supplementation recruited selectively the mTOR protein complex 2. This study provides the first evidence of the physiological relevance of OCT2-mediated tryptophan transport, and its biological consequences on serotonin homeostasis in the brain and SSRI efficacy.

VE-cadherin in arachnoid and pia mater cells serves as a suitable landmark for in vivo imaging of CNS immune surveillance and inflammation

Meninges cover the surface of the brain and spinal cord and contribute to protection and immune surveillance of the central nervous system (CNS). How the meningeal layers establish CNS compartments with different accessibility to immune cells and immune mediators is, however, not well understood. Here, using 2-photon imaging in female transgenic reporter mice, we describe VE-cadherin at intercellular junctions of arachnoid and pia mater cells that form the leptomeninges and border the subarachnoid space (SAS) filled with cerebrospinal fluid (CSF). VE-cadherin expression also marked a layer of Prox1+ cells located within the arachnoid beneath and separate from E-cadherin+ arachnoid barrier cells. In vivo imaging of the spinal cord and brain in female VE-cadherin-GFP reporter mice allowed for direct observation of accessibility of CSF derived tracers and T cells into the SAS bordered by the arachnoid and pia mater during health and neuroinflammation, and detection of volume changes of the SAS during CNS pathology. Together, the findings identified VE-cadherin as an informative landmark for in vivo imaging of the leptomeninges that can be used to visualize the borders of the SAS and thus potential barrier properties of the leptomeninges in controlling access of immune mediators and immune cells into the CNS during health and neuroinflammation.

Regional Differences in the Absolute Abundance of Transporters, Receptors and Tight Junction Molecules at the Blood-Arachnoid Barrier and Blood-Spinal Cord Barrier among Cervical, Thoracic and Lumbar Spines in Dogs

PURPOSE

The purpose of the present study was to quantitatively determine the expression of transporters, receptors and tight junction molecules at the blood-arachnoid barrier (BAB) and blood-spinal cord barrier (BSCB) in cervical, thoracic and lumbar spines from dogs.

METHODS

The expression levels of 31 transporters, 3 receptors, 1 tight junction protein, and 3 marker proteins in leptomeninges and capillaries isolated from spines (3 male and 2 female dogs) were determined by quantitative Targeted Absolute Proteomics (qTAP). The units were converted from fmol/μg protein to pmol/cm (absolute abundance at the BAB and the BSCB in a 1 cm section of spine).

RESULTS

The expression of MDR1 and BCRP were greater at the BSCB compared to the BAB (especially in the cervical cord), and the expressions at the lumbar BSCB were lower than that for the cervical BSCB. Among the organic anionic and cationic drug transporters, OAT1, OAT3, MRP1, OCT2 and MATE1/2 were detected only in the BAB, and not at the BSCB). The expression of these transporters was higher in the order: lumbar > thoracic > cervical BAB. The expressions of GLUT1, 4F2hc, EAAT1, 2, PEPT2, CTL1, and MCT1 at the BSCB of the cervical cord were higher than the corresponding values for the cervical BAB, and these values decreased in going down the spinal cord.

CONCLUSION

These results provide a better understanding of the molecular mechanisms underlying the concentration gradients of drugs and endogenous substances in the cerebrospinal fluid and parenchyma of the spinal cord.