Accumulation of peptide Tyr-D-Ala-Gly by choroid plexus during ventriculocisternal perfusion of rat brain

Role of PEPT2 in peptide/mimetic trafficking at the blood-cerebrospinal fluid barrier: studies in rat choroid plexus epithelial cells in primary culture

Recent studies have established the functional and molecular presence of a high-affinity peptide transporter, PEPT2, in whole tissue rat choroid plexus. However, the precise membrane location and directionality of PEPT2-mediated transport is uncertain at present. In this study, we examined the transport kinetics of a model dipeptide, glycylsarcosine (GlySar), along with the protein expression of PEPT2 using primary cell cultures of choroidal epithelium from neonatal rats. GlySar accumulation and transepithelial transport were 3 to 4 times higher when introduced from the apical as opposed to the basal side of the monolayers. GlySar apical uptake was also stimulated by an inwardly directed proton gradient. The uptake of GlySar was inhibited by di/tripeptides, carnosine, and alpha-amino cephalosporins but was unaffected by amino acids, cephalosporins lacking an alpha-amino group, and organic anions and cations. The Michaelis constant (K(m)) of GlySar was 59.6 microM for apical uptake and 1.4 mM for basal uptake; this is consistent with the high-affinity properties of PEPT2 at the apical membrane. Immunoblot analyses and immunofluorescent confocal microscopy demonstrated the presence of PEPT2, but not PEPT1, in rat choroid plexus epithelial cells. Moreover, PEPT2 was present in the apical and subapical regions of the cell but was absent in the basolateral membrane. These findings demonstrate, for the first time, that PEPT2 protein is present at the apical membrane of choroidal epithelial cells and that it is functionally active at this membrane surface. The results suggest that PEPT2 may have a role in the efflux of peptides and/or mimetics from cerebrospinal fluid to the blood.

PEPT2-mediated uptake of neuropeptides in rat choroid plexus

PURPOSE

The peptide transporter PEPT2 was recently shown to be functionally active in rat choroid plexus, suggesting that it may play a role in neuropeptide homeostasis in the cerebrospinal fluid. This study, therefore, examined the role of PEPT2 in mediating neuropeptide uptake into choroid plexus.

METHODS

Whole-tissue rat choroid plexus uptake studies were performed on GlySar in the absence and presence of neuropeptides and on carnosine.

RESULTS

The neuropeptides NAAG, CysGly, GlyGln, kyotorphin, and carnosine inhibited the uptake of radiolabeled GlySar at 1.0 mM concentrations. In contrast, TRH, [D-Arg2]-kyotorphin, glutathione, and homocarnosine did not inhibit GlySar uptake. Kyotorphin, an analgesic, was a competitive inhibitor of GlySar with a Ki of 8.0 microM. The direct uptake of carnosine was also shown to be mediated by PEPT2 in isolated choroid plexus (Km = 39.3 microM; Vmax = 73.9 pmol/mg/min). Radiolabeled carnosine uptake was inhibited by 1.0 mM concentrations of GlySar or carnosine but not homocarnosine, L-histidine, or beta-alanine.

CONCLUSIONS

These findings indicate that PEPT2 mediates the uptake of a diverse group of neuropeptides in choroid plexus, and suggests a role for PEPT2 in the regulation of neuropeptides, peptide fragments, and peptidomimetics in cerebrospinal fluid.

Formycin B elimination from the cerebrospinal fluid of the rat

The goal of this study was to determine whether specific transport systems are involved in nucleoside elimination from the cerebrospinal fluid (CSF). First, in vitro studies were carried out in isolated choroid plexus tissue slices from rat to ascertain the mechanisms of transport of formycin B, a model nucleoside analogue. 3H-Formycin B accumulated against a concentration gradient in the presence of an Na+ gradient in the isolated ATP-depleted choroid plexus tissue slices. This accumulation was reduced by high concentrations of unlabeled formycin B. Nitrobenzylthioinosine (NBMPR), an equilibrative nucleoside transport inhibitor, inhibited the uptake of formycin B in the absence of an Na+ gradient. These data suggest that both equilibrative and secondary active Na(+)-nucleoside transport systems are present in rat choroid plexus. In vivo, formycin B, together with inulin as a bulk flow marker, was injected into the lateral ventricle of the anesthetized rat with the aid of a stereotaxic device, and CSF was sampled from the cisterna magna at various times after injection. Twelve rats were randomized and divided into a low- and a high-dose group. The CSF clearance (CLCSF) of formycin B was significantly higher than the CLCSF of inulin in both animal groups (P < 0.01), indicating that formycin B is cleared from CSF by a pathway(s) in addition to bulk flow. Formycin B CLCSF was significantly lower in the high-dose group than in the low-dose group (P < 0.05), suggesting a saturable CSF elimination. The CLCSF of formycin B was also significantly reduced in animals treated with NBMPR (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between the blood-brain barrier and endogenous peptides: emerging clinical implications

The effects of peptides on brain function suggest therapeutic and pathologic roles for these substances. Many peptides cross the blood-brain barrier (BBB) by transmembrane diffusion as a function of their lipid solubilities. Other peptides, such as the enkephalins, Tyr-MIF-1, vasopressin-related peptides, and peptide T-like peptides, are transported by carrier-mediated systems. Passage is influenced by aging, stress, lighting, drugs, amino acids, and neurotoxins. Disruption of the BBB results in complex changes in the blood and CSF levels of peptides. Peptides influence the passage of glucose, amino acids, and inorganic acids and may affect the integrity of the BBB. Peptide-BBB interactions have been suggested to play direct roles in dialysis dementia and maple syrup urine disease; they may be expected to be involved in other disorders of the CNS.

Saturable transport of peptides across the blood-brain barrier

Peptides can be transported across the blood-brain barrier by saturable transport systems. One system, characterized with radioactively labeled Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide), is specific for some of the small peptides with an N-terminal tyrosine, including Tyr-MIF-1, the enkephalins, beta-casomorphin, and dynorphin (1-8). Another separate system transports vasopressin-like peptides. The choroid plexus has at least one system distinguishable from those above that is capable of uptake and possibly transport of opiate-like peptides. The possibility of saturable transport of other peptides has been investigated to a varying degree. Specificity, stereo-specificity, saturability, allosteric regulation, modulation by physiologic and pharmacologic manipulations, and noncompetitive inhibition have been demonstrated to occur in peptide transport systems and suggest a role for them in physiology and disease.

Carrier-mediated transport of vasopressin across the blood-brain barrier of the mouse

A brain to blood carrier-mediated transport system for arginine vasopressin (AVP) was investigated in mice after intraventricular injection of iodinated AVP and varying amounts of unlabeled material or candidate inhibitors. Residual activity in the brain detected after decapitation was used as the main determinant of transport activity. The half-time disappearance of iodinated AVP from the brain was 12.4 min, the Vmax was 1.41 nmol/g-min, and the apparent Km was 28.7 nmol/g. A 30-nmol dose of AVP, mesotocin, arginine vasotocin, pressinoic amide, pressinoic acid, tocinoic acid, and lysine vasotocin, but not oxytocin, lysine vasopressin, AVP free acid, tocinoic amide, Tyr-MIF-1, or cyclo Leu-Gly, significantly (P less than 0.05) inhibited the transport of iodinated AVP out of the brain. The 30 nmol dose of AVP had no effect on the transport of iodide or iodotyrosine out of the brain. High-performance liquid chromatography showed that 59.2% of the radioactivity found in the blood 2 min after an i.c.v. injection of labeled AVP eluted at the same position as labeled AVP compared with 68.8% of radioactivity eluting at that position after material was infused i.v. for 2 min. This indicates that intact peptide is transported across the blood-brain barrier and that most of the degradation of AVP occurs during circulation in the blood. Calculations based on the appearance of radioactivity in the periphery showed that 56.2% of the material injected centrally would have been transported into the periphery by 10 min. This appearance of material in the periphery was inhibited by the simultaneous injection of an excess of unlabeled peptide. Water loading significantly decreased the brain to blood transport rate of AVP by 40%. It is concluded that a saturable system exists for brain to blood transport of AVP and some structurally similar peptides.