Glucose Transporter 1 and Monocarboxylate Transporters 1, 2, and 4 Localization within the Glial Cells of Shark Blood-Brain-Barriers

Carolina Balmaceda-Aguilera,Bruno Peruzzo,Karina Oyarce,Christian Cortés-Campos,María Angeles García,Juan Carlos Tapia,Francisco Nualart,Lauren Mack,Manuel Cifuentes,Christopher R. Weber

doi:10.1371/journal.pone.0032409

Carolina Balmaceda-Aguilera, Bruno Peruzzo + Show 8 more

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https://doi.org/10.1371/journal.pone.0032409

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Abstract

Although previous studies showed that glucose is used to support the metabolic activity of the cartilaginous fish brain, the distribution and expression levels of glucose transporter (GLUT) isoforms remained undetermined. Optic/ultrastructural immunohistochemistry approaches were used to determine the expression of GLUT1 in the glial blood-brain barrier (gBBB). GLUT1 was observed solely in glial cells; it was primarily located in end-feet processes of the gBBB. Western blot analysis showed a protein with a molecular mass of 50 kDa, and partial sequencing confirmed GLUT1 identity. Similar approaches were used to demonstrate increased GLUT1 polarization to both apical and basolateral membranes in choroid plexus epithelial cells. To explore monocarboxylate transporter (MCT) involvement in shark brain metabolism, the expression of MCTs was analyzed. MCT1, 2 and 4 were expressed in endothelial cells; however, only MCT1 and MCT4 were present in glial cells. In neurons, MCT2 was localized at the cell membrane whereas MCT1 was detected within mitochondria. Previous studies demonstrated that hypoxia modified GLUT and MCT expression in mammalian brain cells, which was mediated by the transcription factor, hypoxia inducible factor-1. Similarly, we observed that hypoxia modified MCT1 cellular distribution and MCT4 expression in shark telencephalic area and brain stem, confirming the role of these transporters in hypoxia adaptation. Finally, using three-dimensional ultrastructural microscopy, the interaction between glial end-feet and leaky blood vessels of shark brain was assessed in the present study. These data suggested that the brains of shark may take up glucose from blood using a different mechanism than that used by mammalian brains, which may induce astrocyte-neuron lactate shuttling and metabolic coupling as observed in mammalian brain. Our data suggested that the structural conditions and expression patterns of GLUT1, MCT1, MCT2 and MCT4 in shark brain may establish the molecular foundation of metabolic coupling between glia and neurons.

Highlights

Based on functional studies in a limited number of submammalian vertebrates, it appears that the physiological characteristics of the blood-brain-barriers (BBBs) are similar among the vertebrate classes [1]
Our data suggested that the structural conditions and expression patterns of GLUT1, MCT1, MCT2 and MCT4 in shark brain may establish the molecular foundation of metabolic coupling between glia and neurons
The expression and distribution of the glucose transporter, GLUT1, at both the optical and ultrastructural levels was characterized in the radial glial cells that make up the BBB and the epithelial cells that form the blood-cerebrospinal fluid (CSF) barrier in shark brain

Summary

Introduction

Based on functional studies in a limited number of submammalian vertebrates (e.g., primarily jawless vertebrates, cartilaginous fishes, and amphibians), it appears that the physiological characteristics of the blood-brain-barriers (BBBs) are similar among the vertebrate classes [1]. It is postulated that if the glial cells (i.e., astrocytes) within the BBB of cartilaginous fish participate in the transcellular transport of glucose, they may overexpress molecules involved in carrier-mediated transport mechanisms [6] and metabolic coupling such as glucose transporters (GLUT) and monocarboxylate transporters (MCT) [7,8,9,10]. The general expression and localization of GLUTs has been primarily studied in mammalian species; a detailed analysis in submammalian vertebrates has yet to be performed. Glucose is vital for the metabolic activity of the cartilaginous and bony fishes brain, no immunohistochemical data analyzing the distribution and expression levels of GLUT isoforms involved in the acquisition of glucose exists. Glucose permeability analysis of the BBB in shark (Squalus acanthias) indicated that the transport of 3-O-methyl-glucose is mediated by a saturable and stereospecific component, suggesting the expression of a transporter similar to GLUT1 [6]

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