Reduced permeation of 14C-sucrose, 3H-mannitol and 3H-inulin across blood-brain barrier in nephrectomized rats

LC-MS/MS-based in vitro and in vivo investigation of blood-brain barrier integrity by simultaneous quantitation of mannitol and sucrose

BACKGROUND

Understanding the pathophysiology of the blood brain-barrier (BBB) plays a critical role in diagnosis and treatment of disease conditions. Applying a sensitive and specific LC-MS/MS technique for the measurement of BBB integrity with high precision, we have recently introduced non-radioactive [¹³C₁₂]sucrose as a superior marker substance. Comparison of permeability markers with different molecular weight, but otherwise similar physicochemical properties, can provide insights into the uptake mechanism at the BBB. Mannitol is a small hydrophilic, uncharged molecule that is half the size of sucrose. Previously only radioactive [³H]mannitol or [¹⁴C]mannitol has been used to measure BBB integrity.

METHODS

We developed a UPLC-MS/MS method for simultaneous analysis of stable isotope-labeled sucrose and mannitol. The in vivo BBB permeability of [¹³C₆]mannitol and [¹³C₁₂]sucrose was measured in mice, using [¹³C₆]sucrose as a vascular marker to correct for brain intravascular content. Moreover, a Transwell model with induced pluripotent stem cell-derived brain endothelial cells was used to measure the permeability coefficient of sucrose and mannitol in vitro both under control and compromised (in the presence of IL-1β) conditions.

RESULTS

We found low permeability values for both mannitol and sucrose in vitro (permeability coefficients of 4.99 ± 0.152 × 10^-7 and 3.12 ± 0.176 × 10^-7 cm/s, respectively) and in vivo (PS products of 0.267 ± 0.021 and 0.126 ± 0.025 µl g^-1 min^-1, respectively). Further, the in vitro permeability of both markers substantially increased in the presence of IL-1β. Corrected brain concentrations (C_br), obtained by washout vs. vascular marker correction, were not significantly different for either mannitol (0.071 ± 0.007 and 0.065 ± 0.009 percent injected dose per g) or sucrose (0.035 ± 0.003 and 0.037 ± 0.005 percent injected dose per g). These data also indicate that C_br and PS product values of mannitol were about twice the corresponding values of sucrose.

CONCLUSIONS

We established a highly sensitive, specific and reproducible approach to simultaneously measure the BBB permeability of two classical low molecular weight, hydrophilic markers in a stable isotope labeled format. This method is now available as a tool to quantify BBB permeability in vitro and in vivo in different disease models, as well as for monitoring treatment outcomes.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Effects of hepatic ischemia-reperfusion injury on the blood-brain barrier permeability to [14C] and [13C]sucrose

Hepatic encephalopathy that is associated with severe liver failure may compromise the blood-brain barrier (BBB) integrity. However, the effects of less severe liver diseases, in the absence of overt encephalopathy, on the BBB are not well understood. The goal of the current study was to investigate the effects of hepatic ischemia-reperfusion (IR) injury on the BBB tight junction permeability to small, hydrophilic molecules using the widely used [¹⁴C]sucrose and recently-proposed alternative [¹³C]sucrose as markers. Rats were subjected to 20 min of hepatic ischemia or sham surgery, followed by 8 h of reperfusion before administration of a single bolus dose of [¹⁴C] or [¹³C]sucrose and collection of serial (0-30 min) blood and plasma and terminal brain samples. The concentrations of [¹⁴C] and [¹³C]sucrose in the samples were determined by measurement of total radioactivity (nonspecific) and LC-MS/MS (specific), respectively. IR injury significantly increased the blood, plasma, and brain concentrations of both [¹⁴C] and [¹³C]sucrose. However, when the brain concentrations were corrected for their respective area under the blood concentration-time curve, only [¹⁴C]sucrose showed significantly higher (30%) BBB permeability values in the IR animals. Because [¹³C]sucrose is a more specific BBB permeability marker, these data indicate that our animal model of hepatic IR injury does not affect the BBB tight junction permeability to small, hydrophilic molecules. Methodological differences among studies of the effects of liver diseases on the BBB permeability may confound the conclusions of such studies.

Evaluation of [14C] and [13C]Sucrose as Blood-Brain Barrier Permeability Markers

Nonspecific quantitation of [¹⁴C]sucrose in blood and brain has been routinely used as a quantitative measure of the in vivo blood-brain barrier (BBB) integrity. However, the reported apparent brain uptake clearance (K_in) of the marker varies widely (∼100-fold). We investigated the accuracy of the use of the marker in comparison with a stable isotope of sucrose ([¹³C]sucrose) measured by a specific liquid chromatography-tandem mass spectrometry method. Rats received single doses of each marker, and the K_in values were determined. Surprisingly, the K_in value of [¹³C]sucrose was 6- to 7-fold lower than that of [¹⁴C]sucrose. Chromatographic fractionation after in vivo administration of [¹⁴C]sucrose indicated that the majority of the brain content of radioactivity belonged to compounds other than the intact [¹⁴C]sucrose. However, mechanistic studies failed to reveal any substantial metabolism of the marker. The octanol:water partition coefficient of [¹⁴C]sucrose was >2-fold higher than that of [¹³C]sucrose, indicating the presence of lipid-soluble impurities in the [¹⁴C]sucrose solution. Our data indicate that [¹⁴C]sucrose overestimates the true BBB permeability to sucrose. We suggest that specific quantitation of the stable isotope (¹³C) of sucrose is a more accurate alternative to the current widespread use of the radioactive sucrose as a BBB marker.

Development and validation of a sensitive UPLC-MS/MS method for the quantitation of [(13)C]sucrose in rat plasma, blood, and brain: Its application to the measurement of blood-brain barrier permeability

Accurate and reproducible measurement of blood-brain barrier (BBB) integrity is critical in the assessment of the pathophysiology of the central nervous system disorders and in monitoring therapeutic effects. The widely-used low molecular weight marker [(14)C]sucrose is non-specific in the absence of chromatographic separation. The purpose of this study was to develop and validate a sensitive and reproducible LC-MS/MS method for the analysis of stable isotope-modified [(13)C12]sucrose in brain, plasma, and blood to determine BBB permeability to sucrose. After addition of internal standard (IS, [(13)C6]sucrose), the marker and IS were recovered from diluted rat blood, plasma, and brain homogenate by protein precipitation using acetonitrile. The recovery of the marker and IS was almost quantitative (90-106%) for all three matrices. The recovered samples were directly injected into an isocratic UPLC system with a run time of 6 min. Mass spectrometry was conducted using multiple reaction monitoring in negative mode. The method was linear (r(2)≥0.99) in the concentration ranges tested for the diluted blood and plasma (10-1000 ng/mL) and brain homogenate (1-200 ng/mL). The lower limit of quantitation of the assay was 0.5 pg injected on column. The assay was validated (n=5) based on acceptable intra- and inter-run accuracy and precision values. The method was successfully used for the measurement of serial blood and plasma and terminal brain concentrations of [(13)C12]sucrose after a single intravenous dose (10 mg/kg) of the marker to rats. As expected, the apparent brain uptake clearance values of [(13)C12]sucrose were low in healthy rats. The method may be useful for determination of the BBB integrity in animal models.

Tight junction protein expression and barrier properties of immortalized mouse brain microvessel endothelial cells

Understanding the molecular and biochemical mechanisms regulating the blood-brain barrier is aided by in vitro model systems. Many studies have used primary cultures of brain microvessel endothelial cells for this purpose. However, primary cultures limit the generation of material for molecular and biochemical assays since cells grow slowly, are prone to contamination by other neurovascular unit cells, and lose blood-brain barrier characteristics when passaged. To address these issues, immortalized cell lines have been generated. In these studies, we assessed the suitability of the immortalized mouse brain endothelial cell line, bEnd3, as a blood-brain barrier model. RT-PCR and immunofluorescence indicated expression of multiple tight junction proteins. bEnd3 cells formed barriers to radiolabeled sucrose, and responded like primary cultures to disrupting stimuli. Exposing cells to serum-free media on their basolateral side significantly decreased paracellular permeability; astrocyte-conditioned media did not enhance barrier properties. The serum-free media-induced decrease in permeability was correlated with an increase in claudin-5 and zonula occludens-1 immunofluorescence at cell-cell contracts. We conclude that bEnd3 cells are an attractive candidate as a model of the blood-brain barrier due to their rapid growth, maintenance of blood-brain barrier characteristics over repeated passages, formation of functional barriers and amenability to numerous molecular interventions.

Quantitative dual-probe microdialysis: evaluation of [3H]mannitol diffusion in agar and rat striatum

Dual-probe microdialysis was used to study interstitial diffusion in the rat brain. A radiolabelled tracer, (3H]mannitol, was continuously infused at different concentrations via a probe acutely implanted into the striatum of an anaesthetized male rat or into a dilute agar gel. Samples were collected by a second probe placed 1 mm away from the first, and the recovered [3H]mannitol was measured by liquid scintillation counting. In the striatum, the delivery of [3H]mannitol was counteracted by its removal from the extracellular space by passive uptake into cells and clearance into the microcirculation, causing the diffusion profile to approach quasi steady-state levels within 2 h. Diffusion data from brain and agar were analysed using a mathematical model. The apparent (effective) diffusion coefficient for [3H]mannitol was D* = 2.9 x 10(-6) cm2/s, the effective volume fraction alpha* = 0.30 and the clearance rate constant kappa= 2.3 x 10(-5)/s. A tortuosity, lambda = 1.81, and penetration distance r = 4.2 mm, were calculated. We conclude that, using dual-probe microdialysis, parameters reflecting geometric and dynamic tissue properties may be obtained using appropriate mathematical analysis. Quantitative dual-probe microdialysis will be valuable in characterizing interstitial diffusion and the clearance processes underpinning volume transmission in the brain.

Transfer constants for blood-brain barrier permeation of the neuroexcitatory shellfish toxin, domoic acid

The cause of the toxic mussel poisoning episode in 1987 was traced to a plankton-produced excitotoxin, domoic acid. Experiments were undertaken to quantitate the degree to which blood-borne domoic acid can permeate the microvasculature to enter the brain. Pentobarbital-anesthetized, adult rats received an i.v. injection of 3H-domoic acid which was permitted to circulate for 3-60 min. Transfer constants (Ki) describing blood-to-brain diffusion of tracer were calculated from analysis of the relationship between brain vs plasma radioactivity with time. Mean values (mL.g-1.s-1 X 10(6] for permeation into 7 brain regions (n = 10 rats) ranged from 1.60 +/- 0.13 (SE) to 1.86 +/- 0.33 (cortex, pons-medulla respectively), and carrier transport or regional selectivity in uptake were not evident. Nephrectomy prior to domoic acid injection resulted in the elevation of circulating plasma tracer level and brain uptake. The Ki values are comparable to those for other polar compounds such as sucrose, and indicate that the blood-brain barrier greatly limits the amount of toxin that enters the brain. Together with absorbed dosage, integrity of the cerebrovascular barrier and normal kidney function are important to the outcome of accidentally ingesting domoic acid.

Inhibition of blood-brain barrier permeability to DL-propranolol by serum from acute renal failure rats

The effect of uranyl nitrate-induced acute renal failure on the brain uptake of DL-propranolol was investigated in rats with a series of tissue-sampling single-carotid injection techniques. When the buffer solution was used as an injection solution, the brain uptake index (BUI), the extraction ratio (ET), and the blood-brain barrier (BBB) permeability-surface area product (PSapp) and PSu (corrected PSapp for the unbound fraction) in uremic rats were significantly lower than those in control rats. These parameters for DL-propranolol were decreased significantly in both control and uremic rats receiving injection of the uremic serum. The PSu values in both of the control and uremic rats injected with either control or uremic rat serum were significantly higher than those in rats injected with the buffer solution, suggesting the presence of a protein-mediated transport mechanism; that is, the conventional assumption that the fraction of the drug which is available for the uptake in vivo is equal to the unbound fraction as measured in vitro may not hold. In contrast, the brain extraction of D-[14C]glucose, [3H]inulin and [3H]water, which show no binding to serum protein, was not affected by the coinjection of either control or uremic rat serum. On the other hand, using either the ultrafiltrate from serum (control and uremic) or supernatant fraction from heat-treated serum (control and uremic) as the injection solution, no significant difference in the PSu value for DL-propranolol was observed between control and uremic serum. These results suggest that (1) the decrease in the PSu value for DL-propranolol in uremic rats may be attributed mainly to the presence of an endogenous inhibitory substance(s) for the brain uptake or to the decrease in the exchangeable fraction in vivo in the uremic serum; (2) the decrease in the PSu value for DL-propranolol may also be partly attributed to the change in the BBB permeability and/or surface area; (3) the inhibitor for the brain uptake may be characterized as a temperature-sensitive and nonfiltrable substance(s) at physiological pH; and (4) the ability of protein-mediated transport for DL-propranolol into brain was decreased in uremic rats.