Developmental Regulation of the Rabbit Blood-Brain Barrier LAT1 Large Neutral Amino Acid Transporter mRNA and Protein

Abstract

The expression of the blood-brain barrier (BBB) LAT1 large neutral amino acid transporter mRNA and protein was investigated in development in rabbits. The BBB LAT1 mRNA was down-regulated with postnatal development. However, the BBB immunoreactive LAT1 protein was unchanged in postnatal development, despite an up-regulation of the BBB GLUT1 glucose transporter protein during this period. The dissociation between LAT1 protein and mRNA levels in development is consistent with posttranscriptional regulation of BBB LAT1 gene expression. The transport of the large neutral amino acids from blood to brain is mediated by the LAT1 large amino acid transporter located at the brain capillary endothelial membrane (1, 2), which forms the BBB in vivo. The LAT1 is also called the leucine-preferring or L-system amino acid transporter (3), and was originally cloned from C6 glial cells (4). The pools of amino acids in blood and in the brain intracellular spaces are separated by two membranes in series: the BBB and the BCM. Owing to the vastly greater surface area of the BCM compared with the BBB (5), the rate-limiting step in brain uptake of circulating amino acids is BBB transport (6). Therefore, amino acid availability in brain is regulated by BBB transport, and the availability of large neutral amino acids in brain regulates brain metabolism. For example, the influx of amino acids from blood to brain approximates the rates of amino acid incorporation into brain proteins (6). Under normal physiologic conditions, the synthesis of brain proteins is not influenced by the availability of amino acids (7). However, this pathway is inhibited when the supply of brain amino acids is altered by a singular hyperaminoacidemia, like phenylketonuria (PKU) (8 -11). PKU causes a decrease in amino acid supply in brain by competition effects at the BBB, and this decrease in brain protein synthesis is normalized by the administration of amino acids (12). In addition, many pathways of brain neurotransmitter synthesis are also rate-affected by the cerebral availability of precursor large neutral amino acids (6). In the developing brain, there are changes in protein synthesis that parallel the general reduction in cellular density of the older brain compared with the fetal or neonatal brain (13,14). Whether there is a parallel down-regulation of the BBB LAT1 gene expression with development is not known, although other BBB-nutrient transporters are developmentally regulated. For example, the gene expression of the BBB GLUT1 glucose transporter mRNA is up-regulated as suckling animals are weaned (15, 16). However, the regulation of the GLUT1 protein at the BBB is not linked to that of the BBB GLUT1 mRNA in developing rabbits, consistent with a posttranscriptional mechanism of regulation of BBB GLUT1 gene expression (16). The BBB GLUT1 glucose transporter protein is initially down-regulated after birth (16), when suckling animals use ketone bodies as a principal carbon source for the brain (15). Later in postnatal development, the BBB GLUT1 transporte...

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“…2004; Manfroi et al. 2004), expresses high LAT1 mRNA levels that are down‐regulated during the postnatal developmental period (Boado et al. 2004).…”

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