GLUT8 is a novel glucose transporter that exhibits significant sequence similarity with the members of the sugar transport facilitator family GLUT. This novel glucose transporter was independently cloned by several research groups (Doege et al. 2000; Ibberson et al. 2000; Carayannopoulos et al. 2002). Human and mouse sequences have 86.2 % identical amino acids and comprise twelve putative membrane-spanning helices and several conserved motifs. GLUT8 is expressed to some extent in insulin-sensitive tissues, e.g. heart, skeletal muscle and adipose tissue. However, the highest levels of GLUT8 mRNA were found in testis and appeared to be hormonally regulated. GLUT8 is also expressed in the small intestine, where the glucose transport is modulated by hormones.

The purpose of the present study was to examine GLUT8 expression in mouse enterocytes and compare this with other tissues, in an attempt to determine GLUT8 contribution to intestinal glucose absorption.

Handling, equipment used, and killing of mice was carried out in accordance with the European Council legislation 86/609/EEC concerning experimental animal protection. After killing, tissues (jejunum, colon, kidney, testis and brain) were removed quickly, and total RNA was isolated. cDNA was synthesised by the Superscript First-Strand Synthesis System for RT-PCR (Invitrogen), after RNA quantification and quality proofing. Mouse GLUT8 exon-intron organisation is known and we designed specifics primers for GLUT8 cDNA amplification. The sense and antisense primers were (5â-TGACATGTCTCCCGAGGACC-3â), and (5â-TTTCCCAGACAGTAGCAGG-3â), both located on exon I and X, respectively. PCR products were cloned using the TOPO cloning system (Invitrogen), and cDNA sequences determined by sequencing. Southern blot analysis was performed to examine the alternative splicing of GLUT8. RT-PCR products generated were treated with 40 U S1 nuclease (Roche), and after agarose (2 %) gel electrophoresis transferred to nitrocellulose, and then hybridised with a GLUT8 cDNA probe.

To study GLUT8 expression on small intestine and to compare with other tissues, RT-PCR screening was performed from total RNA tissues using two sets of primers (amplified from the N-terminus to the C-terminus). We have found that GLUT8 is expressed in all tissues screened (1438 pb), small intestine included. Moreover, in small intestine, in addition to the full length, we have found several alternative splicing transcripts of mouse GLUT8 as a product of specific exon deletion. We do not know yet the specific role of these transcripts in the small intestine, but they could be a specific mechanism in order to regulate GLUT8 expression in this tissue and contribute to the intestinal sugar absorption under specific conditions.

This work was supported in part by grants from Ministerio de Ciencia y Tecnología BFI2002-03590, and from University of Cardenal Herrera-CEU PRUCH02/18.