**1. Introduction**

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The insulin-responsive glucose transporter GLUT4 was first described in 1988 as a result of studies on the regulation of glucose metabolism by insulin [1]. Soon after the discovery of GLUT4, several groups cloned GLUT4 in the human [2], rat [3,4] and mouse [5]. Since its discovery, GLUT4 has received, together with GLUT1, more experimental attention than any other single membrane transport protein. Structurally, GLUT4 follows the predicted model for class I glucose transporters. GLUT4 has a high affinity for glucose, with a Km of approximately 5 mM [6], and also transports mannose, galactose, dehydroascorbic acid and glucosamine [7-10]. In mammals, GLUT4 is mainly expressed in cardiac and skeletal muscle, brown and white adipose tissue, and brain [6,11,12]. GLUT4 plays a pivotal role in whole body glucose homeostasis, mediating the uptake of glucose regulated by insulin [13,14]. GLUT4 is responsible for the reduction in the postprandial rise in plasma glucose levels [6]. Insulin acts by stimulating the translocation of specific GLUT4-containing vesicles from intracellular stores to the plasma membrane (PM) resulting in an immediate increase in glucose transport [6,15]. The disruption of GLUT4 expression has been extensively associat‐ ed with pathologies of impaired glucose uptake and insulin resistance such as type 2 diabetes and obesity [13,16-18].

Glucose is a central molecule for the metabolism of all vertebrates and plays a pivotal role as fuel and metabolic substrate [19]. From an evolutionary perspective, the regulation of glucose metabolism in non-mammalian vertebrates appears to be slightly different than in

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mammals, particularly with respect to their tolerance of hyperglycemia (reviewed in [20]). A large body of research has been devoted to the study of glucose metabolism in fish species, mostly due to their phylogenetic position and their economic interest (i.e. fisheries and aquaculture). As a result of their lower ability to return to normoglycemia after feeding or after a glucose load, when compared to mammals, fish have been considered to be "glu‐ cose intolerant" [21]. This conclusion was based on the demonstrated lower ability of fish peripheral tissues (e.g. mostly the skeletal muscle, representing more than 50% of the body weight) to utilize glucose [22]. However, despite the low glucose uptake in the fish skele‐ tal muscle, when compared to mammals, there was evidence indicating that skeletal muscle represents not only the main site of glucose uptake but that it was the only tissue in which the rate of glucose uptake was increased after a glucose load [23], suggesting that glucose uptake in skeletal muscle could be regulated. Given that insulin had been shown to lower blood glucose levels in fish by, at least in part, stimulating the *in vivo* uptake and utiliza‐ tion of glucose mostly by the skeletal muscle [24] and given that the blood levels of insulin were shown to be increased by administration of a glucose load in trout [23], it was hypothesized that insulin may exert its hypoglycemic effects in fish through the regulation of glucose transporters, possibly GLUT4. Initially it was claimed that GLUT4 did not exist in fish based on the inability of antibodies against mammalian GLUT4 to recognize a putative fish GLUT4 protein [25]. However, the identification of a true GLUT4 homolog in fish in 2000 by molecular cloning [26] paved the way for investigating the possible regulation of glucose homeostasis in fish through the regulation of an insulin-regulatable glucose transporter homologous to GLUT4. Here, we review the accumulated evidence to date on the functional characteristics and regulation of the GLUT4 homolog in fish.

**Figure 1.** Evolutionary tree of the GLUT4 gene (SLC2A4) in vertebrates. Alignment of the different GLUT4 gene se‐

Structural and Functional Evolution of Glucose Transporter 4 (GLUT4): A Look at GLUT4 in Fish

http://dx.doi.org/10.5772/58094

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Fish, after mammals, are the vertebrate group for which more genomic information is available mainly due to their importance as research model species (e.g. medaka, zebrafish, stickleback) and for their economic importance in fisheries and aquaculture (e.g. cod, tilapia). Therefore, fish are only second to mammals in the number of species for which there is genomic infor‐ mation corresponding to the GLUT4 gene. In mammals, the GLUT4 gene is located in chro‐ mosome 17 and consists of 11 coding exons, spanning 6.6 kb. Despite the fact that the fish GLUT4 protein is well conserved among the different fish species (see section 3), certain structural differences among the fish GLUT4 genes can be observed. For example, the number of exons coding for GLUT4 and the length of the GLUT4 gene varies among the different species: 12 exons in Fugu, 13 in Tetraodon, 13 in Tilapia, 11 in Stickleback, 12 in Medaka and 15 in Platyfish, spanning 4.8 kb, 4.5 kb, 10.2 kb, 4.9 kb, 12.7 kb and 14.1 kb, respectively (Figure 2). Moreover, it is worth mentioning that exons 6 to 10 of the human GLUT4 gene appear to be highly conserved in fish (Figure 2). The only fish species with a sequenced genome for which the GLUT4 gene appears to be absent is the zebrafish. The lack of a GLUT4 gene in the zebrafish genome sequence database (Ensembl) is related to the lack of transcripts for GLUT4 in zebrafish EST dabases. These observations support the notion that zebrafish may have lost the

In addition to the gene structure characteristics of GLUT4 in fish, another aspect of interest with regards to the evolution of GLUT4 is whether the genes flanking the GLUT4 loci in the genome of the various fish species have also been conserved. A synthetic analysis of the GLUT4 loci in different fish species evidences that a number of the genes flanking the fish GLUT4 genes are homologs to those flanking the human GLUT4 gene (e.g. YBX2, EIF5AL1, GPS2, GABARAP, CTDNEP1, NEURL4, TNK1, PLSCR3) (Figure 3A). Furthermore, the nature and the genomic position of the genes flanking the GLUT4 gene in fish are highly conserved across

quences is shown on the right. White sections of the alignment indicate gaps in the sequence.

**2.1. Structural characteristics of the GLUT4 gene in fish**

GLUT4 gene.

the different fish species (Figure 3B).
