Blood-brain borders: a proposal to address limitations of historical blood-brain barrier terminology.

Many neuroscientists use the term Blood-Brain Barrier (BBB) to emphasize restrictiveness, often equating or reducing the notion of BBB properties to tight junction molecules physically sealing cerebral endothelial cells, rather than pointing out the complexity of this biological interface with respect to its selectivity and variety of exchange between the general blood circulation and the central nervous tissue. Several authors in the field find it unfortunate that the exquisitely dynamic interfaces between blood and brain continue to be viewed primarily as obstructive barriers to transport. Although the term blood-brain interface is an excellent descriptor that does not convey the idea of a barrier, it is important and preferable for the spreading of an idea beyond specialist communities to try to appeal to well-chosen metaphors. Recent evidence reviewed here indicates that blood-brain interfaces are more than selective semi-permeable membranes in that they display many dynamic processes and complex mechanisms for communication. They are thus more like 'geopolitical borders'. Furthermore, some authors working on blood-brain interface-relevant issues have started to use the word border, for example in border-associated macrophages. Therefore, we suggest adopting the term Blood-Brain Border to better communicate the flexibility of and movement across blood-brain interfaces.

Vascular network expansion, integrity of blood-brain interfaces, and cerebrospinal fluid cytokine concentration during postnatal development in the normal and jaundiced rat.

BACKGROUND: Severe neonatal jaundice resulting from elevated levels of unconjugated bilirubin in the blood induces dramatic neurological impairment. Central oxidative stress and an inflammatory response have been associated with the pathophysiological mechanism. Cells forming the blood-brain barrier and the choroidal blood-CSF barrier are the first CNS cells exposed to increased plasma levels of unconjugated bilirubin. These barriers are key regulators of brain homeostasis and require active oxidative metabolism to fulfill their protective functions. The choroid plexus-CSF system is involved in neuroinflammatory processes. In this paper, we address the impact of neonatal hyperbilirubinemia on some aspects of brain barriers. We describe physiological changes in the neurovascular network, blood-brain/CSF barriers integrities, and CSF cytokine levels during the postnatal period in normobilirubinemic animals, and analyze these parameters in parallel in Gunn rats that are deficient in bilirubin catabolism and develop postnatal hyperbilirubinemia. METHODS: Gunn rats bearing a mutation in UGT1a genes were used. The neurovascular network was analyzed by immunofluorescence stereomicroscopy. The integrity of the barriers was evaluated by [14C]-sucrose permeability measurement. CSF cytokine levels were measured by multiplex immunoassay. The choroid plexus-CSF system response to an inflammatory challenge was assessed by enumerating CSF leukocytes. RESULTS: In normobilirubinemic animals, the neurovascular network expands postnatally and displays stage-specific regional variations in its complexity. Network expansion is not affected by hyperbilirubinemia. Permeability of the blood-brain and blood-CSF barriers to sucrose decreases between one- and 9-day-old animals, and does not differ between normobilirubinemic and hyperbilirubinemic rats. Cytokine profiles differ between CSF and plasma in all 1-, 9-, and 18-day-old animals. The CSF cytokine profile in 1-day-old animals is markedly different from that established in older animals. Hyperbilirubinemia perturbs these cytokine profiles only to a very limited extent, and reduces CSF immune cell infiltration triggered by systemic exposure to a bacterial lipopeptide. CONCLUSION: The data highlight developmental specificities of the blood-brain barrier organization and of CSF cytokine content. They also indicate that a direct effect of bilirubin on the vascular system organization, brain barriers morphological integrity, and inflammatory response of the choroid plexus-CSF system is not involved in the alteration of brain functions induced by severe neonatal jaundice.

Cerebral concentration and toxicity of endocrine disrupting chemicals: The implication of blood-brain interfaces.

Endocrine disrupting chemicals (EDCs) act on peripheral endocrine organs or interfere with general endocrine pathways. Several EDCs alter the central regulation of neuroendocrine pathways, and affect neurological functions, and as such can be classified as neurotoxic molecules. Environmental pollutants classified as EDCs and affecting the central nervous system include perfluoroalcanes, parabens, phthalates, organotins, bisphenols, benzophenones, polychlorinated biphenyls, and dioxins. In this review we provide a brief description of these families of EDCs. We report and compare the EDC concentrations measured in the brain of humans and wild animals naturally exposed to these molecules, as well as in the brain of laboratory animals experimentally exposed to EDCs. The importance of using sophisticated analytical tools to detect EDCs in the brain is pointed out. The ability of blood-brain interfaces to reduce the brain exposition to EDCs in adult and during development is discussed in relation with the specific morphological, transport and metabolic properties of these cellular layers. Finally, we review the evidence that the neuroprotective functions of blood-brain interfaces can be altered by EDCs, a process that may participate to the central toxic action of these molecules. Overall this analysis points to the implication of blood-brain interfaces in setting the extent of central EDCs toxicity, although most evidence is indirect. Therefore, more specific blood-brain interface-oriented studies are called for in this field of EDC neurotoxicology.

Proteoglycan 4 Reduces Neuroinflammation and Protects the Blood-Brain Barrier after Traumatic Brain Injury.

Neuroinflammation and dysfunction of the blood-brain barrier (BBB) are two prominent mechanisms of secondary injury in neurotrauma. It has been suggested that Toll-like receptors (TLRs) play important roles in initiating and propagating neuroinflammation resulting from traumatic brain injury (TBI), but potential beneficial effects of targeting these receptors in TBI have not been broadly studied. Here, we investigated the effect of targeting TLRs with proteoglycan 4 (PRG4) on post-traumatic neuroinflammation and BBB function. PRG4 is a mucinous glycoprotein with strong anti-inflammatory properties, exerting its biological effects by interfering with TLR2/4 signaling. In addition, PRG4 has the ability to inhibit activation of cluster of differentiation 44 (CD44), a cell-surface glycoprotein playing an important role in inflammation. Using the controlled cortical impact model of TBI in rats, we showed a rapid and prolonged upregulation of message for TLR2/4 and CD44 in the injured cortex. In the in vitro model of the BBB, recombinant human PRG4 (rhPRG4) crossed the endothelial monolayers through a high-capacity, saturable transport system. In rats sustaining TBI, PRG4 delivery to the brain was enhanced by post-traumatic increase in BBB permeability. rhPRG4 injected intravenously at 1 h post-TBI potently inhibited post-traumatic activation of nuclear factor kappa B and extracellular signal-regulated kinases 1/2, the two major signal transduction pathways associated with TLR2/4 and CD44, and curtailed the post-traumatic influx of monocytes. In addition, PRG4 restored normal BBB function after TBI by preventing the post-traumatic loss of tight junction protein claudin 5 and reduced neuronal death. Our observations provide support for therapeutic strategies targeting TLRs in TBI.

Purified IgG from aquaporin-4 neuromyelitis optica spectrum disorder patients alters blood-brain barrier permeability.

BACKGROUND: Neuromyelitis optica spectrum disorders (NMOSD) is a primary astrocytopathy driven by antibodies directed against the aquaporin-4 water channel located at the end-feet of the astrocyte. Although blood-brain barrier (BBB) breakdown is considered one of the key steps for the development and lesion formation, little is known about the molecular mechanisms involved. The aim of the study was to evaluate the effect of human immunoglobulins from NMOSD patients (NMO-IgG) on BBB properties. METHODS: Freshly isolated brain microvessels (IBMs) from rat brains were used as a study model. At first, analysis of the secretome profile from IBMs exposed to purified NMO-IgG, to healthy donor IgG (Control-IgG), or non-treated, was performed. Second, tight junction (TJ) proteins expression in fresh IBMs and primary cultures of brain microvascular endothelial cells (BMEC) was analysed by Western blotting (Wb) after exposition to NMO-IgG and Control-IgG. Finally, functional BBB properties were investigated evaluating the presence of rat-IgG in tissue lysate from brain using Wb in the rat-model, and the passage of NMO-IgG and sucrose in a bicameral model. RESULTS: We found that NMO-IgG induces functional and morphological BBB changes, including: 1) increase of pro-inflammatory cytokines production (CXCL-10 [IP-10], IL-6, IL-1RA, IL-1ß and CXCL-3) in IBMs when exposed to NMO-IgG; 2) decrease of Claudin-5 levels by 25.6% after treatment of fresh IBMs by NMO-IgG compared to Control-IgG (p = 0.002), and similarly, decrease of Claudin-5 by at least 20% when BMEC were cultured with NMO-IgG from five different patients; 3) a higher level of rat-IgG accumulated in periventricular regions of NMO-rats compared to Control-rats and an increase in the permeability of BBB after NMO-IgG treatment in the bicameral model. CONCLUSION: Human NMO-IgG induces both structural and functional alterations of BBB properties, suggesting a direct role of NMO-IgG on modulation of BBB permeability in NMOSD.

N-acetylcysteine inhibits bacterial lipopeptide-mediated neutrophil transmigration through the choroid plexus in the developing brain.

The etiology of neurological impairments associated with prematurity and other perinatal complications often involves an infectious or pro-inflammatory component. The use of antioxidant molecules have proved useful to protect the neonatal brain from injury. The choroid plexuses-CSF system shapes the central nervous system response to inflammation at the adult stage, but little is known on the neuroimmune interactions that take place at the choroidal blood-CSF barrier during development. We previously described that peripheral administration to neonatal mice of the TLR2 ligand PAM3CSK4 (P3C), a prototypic Gram-positive bacterial lipopeptide, induces the migration of innate immune cells to the CSF. Here we showed in neonatal rats exposed to P3C that the migration of neutrophils into the CSF, which occurred through the choroid plexuses, is abolished following administration of the antioxidant drug N-acetylcysteine. Combining light sheet microscopy imaging of choroid plexus, a differentiated model of the blood-CSF barrier, and multiplex cytokine assays, we showed that the choroidal epithelium responds to the bacterial insult by a specific pattern of cytokine secretion, leading to a selective accumulation of neutrophils in the choroid plexus and to their trafficking into CSF. N-acetylcysteine acted by blocking neutrophil migration across both the endothelium of choroidal stromal vessels and the epithelium forming the blood-CSF barrier, without interfering with neutrophil blood count, neutrophil tropism for choroid plexus, and choroidal chemokine-driven chemotaxis. N-acetylcysteine reduced the injury induced by hypoxia-ischemia in P3C-sensitized neonatal rats. Overall, the data show that a double endothelial and epithelial check point controls the transchoroidal migration of neutrophils into the developing brain. They also point to the efficacy of N-acetylcysteine in reducing the deleterious effects of inflammation-associated perinatal injuries by a previously undescribed mechanism, i.e. the inhibition of innate immune cell migration across the choroid plexuses, without interfering with the systemic inflammatory response to infection.

The legacy of Malcolm Beverley Segal (1937-2019) on the science and fields concerned with choroid plexus and cerebrospinal fluid physiology.

This article highlights the scientific achievements, professional career, and personal interactions of Malcolm B. Segal who passed away in July this year. Born in 1937 in Goodmayes, Essex, UK, Segal rose to the Chairman position in the Division of Physiology at United Medical and Dental School of Guy's and St. Thomas' Hospitals, retiring in 2006 after his long professional career in biomedical science. Being trained in Hugh Davson's laboratory, Segal became one of the pioneers in research on cerebrospinal fluid physiology and the choroid plexus. During the course of his career, Segal himself trained a number of young scientists and collaborated with many colleagues around the world, making long-lasting friendships along the way. In addition to his professional accomplishments as a researcher and educator, Segal was an avid sailor and wine connoisseur, and enjoyed teaching classes on navigation and wine tasting.

Developmental differences in the expression of ABC transporters at rat brain barrier interfaces following chronic exposure to diallyl sulfide.

Many pregnant women and prematurely born infants require medication for clinical conditions including cancer, cardiac defects and psychiatric disorders. In adults drug transfer from blood into brain is mostly restricted by efflux mechanisms (ATP-binding cassette, ABC transporters). These mechanisms have been little studied during brain development. Here expression of eight ABC transporters (abcb1a, abcb1b, abcg2, abcc1, abcc2, abcc3, abcc4, abcc5) and activity of conjugating enzyme glutathione-s-transferase (GST) were measured in livers, brain cortices (blood-brain-barrier) and choroid plexuses (blood-cerebrospinal fluid, CSF, barrier) during postnatal rat development. Controls were compared to animals chronically injected (4 days, 200 mg/kg/day) with known abcb1a inducer diallyl sulfide (DAS). Results reveal both tissue- and age-dependent regulation. In liver abcb1a and abcc3 were up-regulated at all ages. In cortex abcb1a/b, abcg2 and abcc4/abcc5 were up-regulated in adults only, while in choroid plexus abcb1a and abcc2 were up-regulated only at P14. DAS treatment increased GST activity in livers, but not in cortex or choroid plexuses. Immunocytochemistry of ABC transporters at the CSF-brain interface showed that PGP and BCRP predominated in neuroepithelium while MRP2/4/5 were prominent in adult ependyma. These results indicate an age-related capacity of brain barriers to dynamically regulate their defence mechanisms when chronically challenged by xenobiotic compounds.

Clinical Imaging of Choroid Plexus in Health and in Brain Disorders: A Mini-Review.

The choroid plexuses (ChPs) perform indispensable functions for the development, maintenance and functioning of the brain. Although they have gained considerable interest in the last years, their involvement in brain disorders is still largely unknown, notably because their deep location inside the brain hampers non-invasive investigations. Imaging tools have become instrumental to the diagnosis and pathophysiological study of neurological and neuropsychiatric diseases. This review summarizes the knowledge that has been gathered from the clinical imaging of ChPs in health and brain disorders not related to ChP pathologies. Results are discussed in the light of pre-clinical imaging studies. As seen in this review, to date, most clinical imaging studies of ChPs have used disease-free human subjects to demonstrate the value of different imaging biomarkers (ChP size, perfusion/permeability, glucose metabolism, inflammation), sometimes combined with the study of normal aging. Although very few studies have actually tested the value of ChP imaging biomarkers in patients with brain disorders, these pioneer studies identified ChP changes that are promising data for a better understanding and follow-up of diseases such as schizophrenia, epilepsy and Alzheimer's disease. Imaging of immune cell trafficking at the ChPs has remained limited to pre-clinical studies so far but has the potential to be translated in patients for example using MRI coupled with the injection of iron oxide nanoparticles. Future investigations should aim at confirming and extending these findings and at developing translational molecular imaging tools for bridging the gap between basic molecular and cellular neuroscience and clinical research.

Choroid plexus glutathione peroxidases are instrumental in protecting the brain fluid environment from hydroperoxides during postnatal development.

Hydrogen peroxide, released at low physiological concentration, is involved in different cell signaling pathways during brain development. When released at supraphysiological concentrations in brain fluids following an inflammatory, hypoxic, or toxic stress, it can initiate lipid peroxidation, protein, and nucleic acid damage and contribute to long-term neurological impairment associated with perinatal diseases. We found high glutathione peroxidase and glutathione reductase enzymatic activities in both lateral and fourth ventricle choroid plexus tissue isolated from developing rats, in comparison to the cerebral cortex and liver. Consistent with these, a high protein expression of glutathione peroxidases 1 and 4 was observed in choroid plexus epithelial cells, which form the blood-cerebrospinal fluid barrier. Live choroid plexuses isolated from newborn rats were highly efficient in detoxifying H2O2 from mock cerebrospinal fluid, illustrating the capacity of the choroid plexuses to control H2O2 concentration in the ventricular system of the brain. We used a differentiated cellular model of the blood-cerebrospinal fluid barrier coupled to kinetic and inhibition analyses to show that glutathione peroxidases are more potent than catalase to detoxify extracellular H2O2 at concentrations up to 250 µM. The choroidal cells also formed an enzymatic barrier preventing blood-borne hydroperoxides to reach the cerebrospinal fluid. These data point out the choroid plexuses as key structures in the control of hydroperoxide levels in the cerebral fluid environment during development, at a time when the protective glial cell network is still immature. Glutathione peroxidases are the main effectors of this choroidal hydroperoxide inactivation.

Barriers to Drug Distribution into the Perinatal and Postnatal Brain.

Drug bioavailability to the developing brain is a major concern in the treatment of neonates and infants as well as pregnant and breast-feeding women. Central adverse drug reactions can have dramatic consequences for brain development, leading to major neurological impairment. Factors setting the cerebral bioavailability of drugs include protein-unbound drug concentration in plasma, local cerebral blood flow, permeability across blood-brain interfaces, binding to neural cells, volume of cerebral fluid compartments, and cerebrospinal fluid secretion rate. Most of these factors change during development, which will affect cerebral drug concentrations. Regarding the impact of blood-brain interfaces, the blood-brain barrier located at the cerebral endothelium and the blood-cerebrospinal fluid barrier located at the choroid plexus epithelium both display a tight phenotype early on in embryos. However, the developmental regulation of some multispecific efflux transporters that also limit the entry of numerous drugs into the brain through barrier cells is expected to favor drug penetration in the neonatal brain. Finally, drug cerebral bioavailability is likely to be affected following perinatal injuries that alter blood-brain interface properties. A thorough investigation of these mechanisms is mandatory for a better risk assessment of drug treatments in pregnant or breast-feeding women, and in neonate and pediatric patients.

Glutathione Conjugation at the Blood-CSF Barrier Efficiently Prevents Exposure of the Developing Brain Fluid Environment to Blood-Borne Reactive Electrophilic Substances.

Exposure of the developing brain to toxins, drugs, or deleterious endogenous compounds during the perinatal period can trigger alterations in cell division, migration, differentiation, and synaptogenesis, leading to lifelong neurological impairment. The brain is protected by cellular barriers acting through multiple mechanisms, some of which are still poorly explored. We used a combination of enzymatic assays, live tissue fluorescence microscopy, and an in vitro cellular model of the blood-CSF barrier to investigate an enzymatic detoxification pathway in the developing male and female rat brain. We show that during the early postnatal period the choroid plexus epithelium forming the blood-CSF barrier and the ependymal cell layer bordering the ventricles harbor a high detoxifying capacity that involves glutathione S-transferases. Using a functional knock-down rat model for choroidal glutathione conjugation, we demonstrate that already in neonates, this metabolic pathway efficiently prevents the penetration of blood-borne reactive compounds into CSF. The versatility of the protective mechanism results from the multiplicity of the glutathione S-transferase isoenzymes, which are differently expressed between the choroidal epithelium and the ependyma. The various isoenzymes display differential substrate specificities, which greatly widen the spectrum of molecules that can be inactivated by this pathway. In conclusion, the blood-CSF barrier and the ependyma are identified as key cellular structures in the CNS to protect the brain fluid environment from different chemical classes of potentially toxic compounds during the postnatal period. This metabolic neuroprotective function of brain interfaces ought to compensate for the liver postnatal immaturity.SIGNIFICANCE STATEMENT Brain homeostasis requires a stable and controlled internal environment. Defective brain protection during the perinatal period can lead to lifelong neurological impairment. We demonstrate that the choroid plexus forming the blood-CSF barrier is a key player in the protection of the developing brain. Glutathione-dependent enzymatic metabolism in the choroidal epithelium inactivates a broad spectrum of noxious compounds, efficiently preventing their penetration into the CSF. A second line of detoxification is located in the ependyma separating the CSF from brain tissue. Our study reveals a novel facet of the mechanisms by which the brain is protected at a period of high vulnerability, at a time when the astrocytic network is still immature and liver xenobiotic metabolism is limited.

Molecular anatomy and functions of the choroidal blood-cerebrospinal fluid barrier in health and disease.

The barrier between the blood and the ventricular cerebrospinal fluid (CSF) is located at the choroid plexuses. At the interface between two circulating fluids, these richly vascularized veil-like structures display a peculiar morphology explained by their developmental origin, and fulfill several functions essential for CNS homeostasis. They form a neuroprotective barrier preventing the accumulation of noxious compounds into the CSF and brain, and secrete CSF, which participates in the maintenance of a stable CNS internal environment. The CSF circulation plays an important role in volume transmission within the developing and adult brain, and CSF compartments are key to the immune surveillance of the CNS. In these contexts, the choroid plexuses are an important source of biologically active molecules involved in brain development, stem cell proliferation and differentiation, and brain repair. By sensing both physiological changes in brain homeostasis and peripheral or central insults such as inflammation, they also act as sentinels for the CNS. Finally, their role in the control of immune cell traffic between the blood and the CSF confers on the choroid plexuses a function in neuroimmune regulation and implicates them in neuroinflammation. The choroid plexuses, therefore, deserve more attention while investigating the pathophysiology of CNS diseases and related comorbidities.

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 bloodbrain 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 addedvalue 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.

T-Lymphocytes Traffic into the Brain across the Blood-CSF Barrier: Evidence Using a Reconstituted Choroid Plexus Epithelium.

An emerging concept of normal brain immune surveillance proposes that recently and moderately activated central memory T lymphocytes enter the central nervous system (CNS) directly into the cerebrospinal fluid (CSF) via the choroid plexus. Within the CSF space, T cells inspect the CNS environment for cognate antigens. This gate of entry into the CNS could also prevail at the initial stage of neuroinflammatory processes. To actually demonstrate T cell migration across the choroidal epithelium forming the blood-CSF barrier, an in vitro model of the rat blood-CSF barrier was established in an "inverse" configuration that enables cell transmigration studies in the basolateral to apical, i.e. blood/stroma to CSF direction. Structural barrier features were evaluated by immunocytochemical analysis of tight junction proteins, functional barrier properties were assessed by measuring the monolayer permeability to sucrose and the active efflux transport of organic anions. The migratory behaviour of activated T cells across the choroidal epithelium was analysed in the presence and absence of chemokines. The migration pathway was examined by confocal microscopy. The inverse rat BCSFB model reproduces the continuous distribution of tight junction proteins at cell margins, the restricted paracellular permeability, and polarized active transport mechanisms, which all contribute to the barrier phenotype in vivo. Using this model, we present experimental evidence of T cell migration across the choroidal epithelium. Cell migration appears to occur via a paracellular route without disrupting the restrictive barrier properties of the epithelial interface. Apical chemokine addition strongly stimulates T cell migration across the choroidal epithelium. The present data provide evidence for the controlled migration of T cells across the blood-CSF barrier into brain. They further indicate that this recruitment route is sensitive to CSF-borne chemokines, extending the relevance of this migration pathway to neuroinflammatory and neuroinfectious disorders which are typified by elevated chemokine levels in CSF.

Novel routes to either racemic or enantiopure α-amino-(4-hydroxy-pyrrolidin-3-yl)acetic acid derivatives and biological evaluation of a new promising pharmacological scaffold.

Cycloaddition between (+) or (-)-menthone-derived nitrones and N-benzyl-3-pyrroline afforded enantiopure spiro-fused heterocycles. The reaction occurred enantio- and diastereo-selectively on the less hindered side of the nitrone, the 3-pyrroline N-benzyl group being oriented outwards, thus controlling the configurations of three simultaneously created chiral centers. From either (+) or (-)-menthone, both enantiomeric cycloadducts were synthesized in excellent yield. Removing the chiral auxiliary and the N-benzyl group delivered a series of enantiopure 4-hydroxy-3-glycinyl-pyrrolidine derivatives in 3-5 steps and 36 to 81 overall yields. Using two other achiral nitrones, shorter routes to racemic analogues were developed. Two of the synthesized compounds markedly lowered extracellular glutamate level and modestly interacted with cannabinoid type-1 receptors. As these two neuroactive compounds were devoid of in vitro toxicity and did not cross the blood brain interface, they might represent potential pharmacological agents to target peripheral organs.

Influx mechanisms in the embryonic and adult rat choroid plexus: a transcriptome study.

The transcriptome of embryonic and adult rat lateral ventricular choroid plexus, using a combination of RNA-Sequencing and microarray data, was analyzed by functional groups of influx transporters, particularly solute carrier (SLC) transporters. RNA-Seq was performed at embryonic day (E) 15 and adult with additional data obtained at intermediate ages from microarray analysis. The largest represented functional group in the embryo was amino acid transporters (twelve) with expression levels 2-98 times greater than in the adult. In contrast, in the adult only six amino acid transporters were up-regulated compared to the embryo and at more modest enrichment levels (<5-fold enrichment above E15). In E15 plexus five glucose transporters, in particular Glut-1, and only one monocarboxylate transporter were enriched compared to the adult, whereas only two glucose transporters but six monocarboxylate transporters in the adult plexus were expressed at higher levels than in embryos. These results are compared with earlier published physiological studies of amino acid and monocarboxylate transport in developing rodents. This comparison shows correlation of high expression of some transporters in the developing brain with higher amino acid transport activity reported previously. Data for divalent metal transporters are also considered. Immunohistochemistry of several transporters (e.g., Slc16a10, a thyroid hormone transporter) gene products was carried out to confirm translational activity and to define cellular distribution of the proteins. Overall the results show that there is substantial expression of numerous influx transporters in the embryonic choroid plexus, many at higher levels than in the adult. This, together with immunohistochemical evidence and data from published physiological transport studies suggests that the choroid plexus in embryonic brain plays a major role in supplying the developing brain with essential nutrients.