Glucose transporter 5 (GLUT5)-like immunoreactivity is localized in subsets of neurons and glia in the rat brain

Highlights

• Distribution of Glut5-like immunoreactivity was investigated in the rat brain.

• Glut5 localized in subsets of neurons and glia.

• Glut5 may exist more widely than one can expect.

Abstract

This study aimed at examining the distribution of glucose transporter 5 (GLUT5), which preferentially transports fructose, in the rat brain by immunohistochemistry and Western blotting. Small immunoreactive puncta (less than 0.7 μm) were sparsely distributed all over the brain, some of which appeared to be associated with microglial processes detected by an anti-ionized calcium-binding adapter molecule 1 (Iba-1) monoclonal antibody. In addition, some of these immunoreactive puncta seemed to be associated with tanycyte processes that were labeled with anti-glial fibrillary acidic protein (GFAP) monoclonal antibody. Ependymal cells were also found to be immunopositive for GLUT5. Furthermore, several noticeable GLUT5 immunoreactive profiles were observed. GLUT5 immunoreactive neurons, confirmed by double staining with neuronal nuclei (NeuN), were seen in the entopeduncular nucleus and lateral hypothalamus. Cerebellar Purkinje cells were immunopositve for GLUT5. Dense accumulation of immunoreactive puncta, some of which were neuronal elements (confirmed by immunoelectron microscopy), were observed in the optic tract and their terminal fields, namely, superior colliculus, pretectum, nucleus of the optic tract, and medial terminal nucleus of the optic tract. In addition to the associated areas of the visual system, the vestibular and cochlear nuclei also contained dense GLUT5 immunoreactive puncta. Western blot analysis of the cerebellum indicated that the antibody used recognized the 33.5 and 37.0 kDa bands that were also contained in jejunum and kidney extracts. Thus, these results suggest that GLUT5 may transport fructose in subsets of the glia and neurons for an energy source of these cells.

Introduction

Glucose is the principal source of energy for the mammalian brain; a continuous supply of this substrate is essential for maintenance of normal brain functions (Maher et al., 1994). Glucose transporter proteins (GLUTs) are highly homologous integral membrane proteins with 12 membrane-spanning domains and a single glycosylation site, and are responsible for the facilitative uptake of glucose and other monosaccharaides into mammalian cells (Bell et al., 1993, Gould and Holman, 1993). To date, 14 GLUT family members have been identified (Mueckler and Thorens, 2013). Among them, GLUT5 (molecular weight, 50–60 kDa in rats; Maher et al., 1994) is highly expressed in the small intestine, kidney, testis and brain (Bell et al., 1993, Gould and Holman, 1993, Cui et al., 2003, Horikoshi et al., 2003). Although GLUT5 is classified as a glucose transporter, it is known to function as a high-affinity fructose transporter with a poor glucose transporting ability (Kayano et al., 1990, Burant et al., 1992, Funari et al., 2007). Most tissues expressing GLUT5 have been found to be rich in fructose, with the exception of the brain (Maher et al., 1994).

Ambient fructose levels are low in the brain (Maher et al., 1994), suggesting the minor contribution of GLUT5 for neuro-energetics. However, a number of studies have shown that GLUT5 is expressed in the Purkinje cells of the cerebellum (Funari et al., 2005), microglia (Payne et al., 1997, Horikoshi et al., 2003) and epithelial cells of choroid plexus, and the ependymal cells in the brain (Ueno et al., 2014). Despite the considerable interest in understanding fructose metabolism in the brain owing to increased consumption of dietary fructose, the overall distribution of GLUT5 in the brain has not been reported so far. In the present study, we investigated the distribution of GLUT5 in the rat brain, and found the more extensive distribution of GLUT5 in subsets of neurons and glia of the rat brain than one might expect.