The steady-state amino acid fluxes across the perfused choroid plexus of the sheep

doi:10.1016/0006-8993(90)90874-B

Brain Research

Volume 525, Issue 2, 20 August 1990, Pages 275-279

https://doi.org/10.1016/0006-8993(90)90874-B Get rights and content

Abstract

The steady-state flux of labelled amino acids was investigated across the isolated perfused choroid plexus of the sheep. The extraction of anionic, cationic, small and large neutral amino acids by the blood side of the choroid plexus was demonstrated. However, there was no uptake of the analogue MeAIB, confirming the absence of the ‘A’ carrier system on this side of the blood-CSF barrier. The direction of the net flux of amino acids across the tissue varied depending on the amino acid and its concentration. At a concentration of 0.01 mM the net movement for phenylalanine, serine, aspartate and glycine was from blood to CSF. When the concentration of amino acid was increased to 0.1 mM, the net flux of phenylalanine and serine remained from blood to CSF whereas the net flux of the transmitters, aspartate and glycine, was in the opposite direction, from CSF to blood. When the level was raised further, to 1 mM, all four amino acids showed a net CSF to blood flux. The concentration of amino acids is newly formed CSF was calculated from the blood to CSF fluxes and was found to be between 2 and 200 μM, similar to that found in mammalian bulk phase CSF.

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The influence of ageing in the cerebrospinal fluid concentrations of proteins that are derived from the choroid plexus, brain, and plasma
2012, Experimental Gerontology
Citation Excerpt :
When respiration ceased, the sheep was rapidly decapitated. After the brain was isolated, the internal carotid arteries were cannulated and the perfusate was directed into the anterior choroidal arteries by ligating all other vessels on the circle of Willis (Preston and Segal, 1990). Each side of the CP was perfused (at a rate of 0.5mLmin− 1 for the middle-aged sheep and 0.9mL min− 1 for the old sheep to give a final flow rate of 3.5 mL/min/g of tissue) with Ringer Tyrode solution containing (in mM): Na+ 145.8, K+ 5.4, HPO42− 0.25, Mg2+ 1.13, HCO3− 26.2, Ca2+ 2.35, Cl− 119.7, glucose 5 and 4% BSA and with washed sheep red cells collected from the same sheep at 200 mL red cells per litre (Preston and Segal, 1990).
Studies have shown that ageing alone can cause increases in the concentrations of many cerebrospinal fluid (CSF) proteins. Therefore, CSF protein concentrations must be interpreted with caution before concluding that the increased concentrations of certain proteins can be used as disease-specific biomarkers. Age-related reduction in CSF turnover has been shown to have a significant concentrating effect on CSF proteins from young to old. As a result, CSF protein concentrations need to be corrected with age-specific turnovers first before performing any data comparisons between different ages. This study applied the concept of CSF/plasma concentration ratios of plasma-derived proteins that is frequently used in the investigation of brain barrier integrity to calculate the amount of protein that enters the CSF from the plasma side in different age groups. Based on our calculations, proteins with molecular weights greater than 91.92 kDa for the young, 109.51 kDa for the middle-aged and 120 kDa for the old should not be able to cross the brain barriers of the blood-brain and blood-CSF barriers to enter the CSF from the plasma side. For proteins that can be derived from the choroid plexus (CP), brain, and plasma, the amount that crosses the barriers to enter the CSF from the plasma side will contribute to their measured total protein concentrations in the CSF. CP and brain production of these proteins can be calculated when turnover corrected CSF protein concentrations are further corrected by the amount of protein that crosses the barriers. In this study, CP and brain produced concentrations of transthyretin, retinol binding protein, alpha-1-antitrypsin, gelsolin, and lactotransferrin were calculated. The production of these proteins decreased with age with alpha-1-antitrypsin protein revealing the most substantial decrease of 86% from young (0.14 ± 0.01 mg^.dL^− 1) to old (0.02 mg^.dL^− 1). In conclusion, measured CSF protein concentrations for proteins that can be derived from the CP, brain, and plasma need to be corrected by age-specific CSF turnovers and by the amount of protein that crosses the brain barriers first before their concentrations can be compared logically between different ages.
Changes in kinetics of amino acid uptake at the ageing ovine blood-cerebrospinal fluid barrier
2012, Neurobiology of Aging
Citation Excerpt :
In the CP, the uptake of Lys was significantly inhibited only by other cationic AA (Preston and Segal, 1992). The Km of the 4 AAs in the young sheep is of the order of 1–10 μM, consistent with our early studies (Preston et al., 1989; Segal et al., 1990; Preston and Segal, 1990, 1992) and approximately within the normal physiological plasma concentration of AA (Smith and Stroll, 1998). System L at the BBB is saturated by endogenous AA under normal conditions as the Km is smaller than the plasma concentration of NAA (Smith and Stroll, 1998).
Amino acids (AA) in brain are precisely controlled by blood–brain barriers, which undergo a host of changes in both morphology and function during ageing. The effect of these age-related changes on AA homeostasis in brain is not well described. This study investigated the kinetics of four AA (Leu, Phe, Ala and Lys) uptakes at young and old ovine choroid plexus (CP), the blood–cerebrospinal fluid (CSF) barrier (BCB), and measured AA concentrations in CSF and plasma samples. In old sheep, the weight of lateral CP increased, so did the ratio of CP/brain. The expansion of the CP is consistent with clinical observation of thicker leptomeninges in old age. AA concentrations in old CSF, plasma and their ratio were different from the young. Both V_max and K_m of Phe and Lys were significant higher compared to the young, indicating higher trans-stimulation in old BCB. Cross-competition and kinetic inhibition studies found the sensitivity and specificity of these transporters were impaired in old BCB. These changes may be the first signs of a compromised barrier system in ageing brain leading increased AA influx into the brain causing neurotoxicity.
The influence of cerebrospinal fluid turnover on age-related changes in cerebrospinal fluid protein concentrations
2010, Neuroscience Letters
Studies have shown that ageing and several neurological diseases of the central nervous system are often accompanied with increase in concentrations of many cerebrospinal fluid (CSF) proteins. However, few studies have actually looked into the mechanisms behind the increase in CSF protein concentrations. In this study, CSF secretion rates and turnovers were measured using the in situ perfused choroid plexus (CP) technique in a group of sheep between 1 and 10 years of age. CSF protein concentrations were determined using quantitative proteomic techniques. CSF turnover in hours correlated significantly with age, changing from 10.5 ± 2.7 h in the young to 17.1 ± 2.4 h in the old. The amount of CSF replaced per hour decreased from 2.46 ± 0.42 mL in the young to 1.17 ± 0.16 mL in the old. The age-related reduction in CSF turnover was calculated to have a concentrating effect of approximately 1.32 times in middle-aged and 2.10 times in old CSF proteins. After CSF turnover normalization, CSF albumin (a plasma-derived protein) concentration still increased significantly with age; however, both brain-derived and partially brain-derived protein concentrations in the CSF decreased with age after normalization. Regression analysis between turnovers and albumin concentrations has shown that reduced CSF turnover was the cause of increased CSF albumin concentrations with age. Therefore, CSF protein concentrations should be normalized according to their age-specific turnovers first before their concentrations can be compared logically between different ages.
Age-related changes in choroid plexus and blood-cerebrospinal fluid barrier function in the sheep
2009, Experimental Gerontology
Dysfunction of the choroid plexuses (CPs) and the blood–cerebrospinal fluid barrier (BCSFB) might contribute to age-related cognitive decline and neurodegenerative disease. We used the CPs from young (1–2 years), middle-aged (3–6 years) and old (7–10 years) sheep to explore effects of aging on various aspects of CP and BCSFB functions. Total protein in the cerebrospinal fluid (CSF) was significantly higher in old compared to young sheep and CSF secretion by the CP perfused in situ was significantly lower in both old and middle-aged when compared to young sheep, which correlated with reduced ²²Na⁺ uptake and efflux by the CP. Steady-state extractions of a low and medium size molecular weight extracellular space marker, ¹⁴C-mannitol and ³H-polyethylene glycol, respectively, were significantly higher in CPs from old compared to young animals; however, there was no significant difference in steady-state extraction of a high molecular weight marker, ¹²⁵I-bovine serum albumin. This indicates increased passive BCSFB permeability for small and medium sized molecules in old sheep. CP redox activity was significantly lower in the old animals as assessed by the MTT assay, however, there was no significant difference in ATP content and energy charge of the CP with age suggesting adequate baseline energy reserve capacity. These data indicate that normal aging processes alter protein content in the CSF, CSF secretion, integrity of the BCSFB and Na⁺ flux in the epithelial layer, which could impact on CSF homeostasis and turnover.
The Choroid Plexus-Cerebrospinal Fluid System: From Development to Aging
2005, Current Topics in Developmental Biology
The function of the cerebrospinal fluid (CSF) and the tissue that secretes it, the choroid plexus (CP), has traditionally been thought of as both providing physical protection to the brain through buoyancy and facilitating the removal of brain metabolites through the bulk drainage of CSF. More recent studies suggest, however, that the CP‐CSF system plays a much more active role in the development, homeostasis, and repair of the central nervous system (CNS). The highly specialized choroidal tissue synthesizes trophic and angiogenic factors, chemorepellents, and carrier proteins, and is strategically positioned within the ventricular cavities to supply the CNS with these biologically active substances. Through polarized transport systems and receptor‐mediated transcytosis across the choroidal epithelium, the CP, a part of the blood‐CSF barrier (BCSFB), controls the entry of nutrients, such as amino acids and nucleosides, and peptide hormones, such as leptin and prolactin, from the periphery into the brain. The CP also plays an important role in the clearance of toxins and drugs.
During CNS development, CP‐derived growth factors, such as members of the transforming growth factor‐β superfamily and retinoic acid, play an important role in controlling the patterning of neuronal differentiation in various brain regions. In the adult CNS, the CP appears to be critically involved in neuronal repair processes and the restoration of the brain microenvironment after traumatic and ischemic brain injury. Furthermore, recent studies suggest that the CP acts as a nursery for neuronal and astrocytic progenitor cells. The advancement of our knowledge of the neuroprotective capabilities of the CP may therefore facilitate the development of novel therapies for ischemic stroke and traumatic brain injury. In the later stages of life, the CP‐CSF axis shows a decline in all aspects of its function, including CSF secretion and protein synthesis, which may in themselves increase the risk for development of late‐life diseases, such as normal pressure hydrocephalus and Alzheimer's disease. The understanding of the mechanisms that underlie the dysfunction of the CP‐CSF system in the elderly may help discover the treatments needed to reverse the negative effects of aging that lead to global CNS failure.
The structure of the choroid plexus and the physiology of the choroid plexus epithelium
2004, Advanced Drug Delivery Reviews
The choroid plexuses (CPs) are leaf-like highly vascular structures laying in the ventricles. The main function of choroid plexuses is the production of the cerebrospinal fluid (CSF). Although CPs have a unique distribution of ion transporters/channels, the mechanism of CSF production is similar to the production of fluids in other epithelia and is based on energy released from ATP hydrolysis, which drives unidirectional flux of ions accompanied by movement of water by osmosis. The CPs have an important role in the homeostasis of nutrients in the CSF since the kinetic parameters of glucose and amino acid (AA) transport across the CPs are the main reason for the low concentration of these molecules in the CSF. The CPs appear to be source of CSF-borne hormones and growth factors, including insulin-like growth factor II (IGF II), vasopressin (VP) and transforming growth factor β1 (TGF-β1). The CPs also synthesise the thyroid transporting protein transthyretin and transferrin and can chelate heavy metals.

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Brain Research

Abstract

Reference (26)

Glycine and alanine concentrations of body fluids: experimental modifications

J. Biol. Chem.

The use of N-methylation to direct the route of mediated transport of amino acids

J. Biol. Chem.

Transport of amino acids by the rabbit choroid plexus in vitro

Brain Research

Flow of interstitial fluid as indicated by the removal of extracellular markers from rat caudate nucleus

Exp. Eye Res.

The steady-state distribution of cycloleucine and AIBA between plasma and cerebrospinal fluid

Exp. Neurol.

Accumulation and transport of amino acids by the frog choroid plexus

Brain Research

Facilitated transport of amino acids through the blood-brain barrier of the dog studied on a single capillary pass

Brain Research

The concentration of amino acids and other electrolytes in cerebrospinal fluid; in vivo dialysate of brain and blood plasma of the dog

J. Neurochem.

Efflux mechanism contributing to the stability of the potassium concentration in cerebrospinal fluid

J. Physiol.

In vitro studies on the uptake and incorporation of neutral amino acids in rabbit choroid plexus

Brain Research

Interorgan amino acid nutrition

Physiol. Rev.

Ventriculo-cisternal perfusion of twelve amino acids in the rabbit

J. Neurobiol.

The Physiology and Pathophysiology of the Cerebrospinal Fluid

Cited by (36)

The influence of ageing in the cerebrospinal fluid concentrations of proteins that are derived from the choroid plexus, brain, and plasma

Changes in kinetics of amino acid uptake at the ageing ovine blood-cerebrospinal fluid barrier

The influence of cerebrospinal fluid turnover on age-related changes in cerebrospinal fluid protein concentrations

Age-related changes in choroid plexus and blood-cerebrospinal fluid barrier function in the sheep

The Choroid Plexus-Cerebrospinal Fluid System: From Development to Aging

The structure of the choroid plexus and the physiology of the choroid plexus epithelium