*4.3.1. GLUT3 physiology*

glucose. Glucose stimulated insulin secretion in islets was impaired by loss of the first, but not the second phase of insulin secretion. GLUT2 knockout mice show marked hyperglucagone‐ mia, and this is accompanied by alterations in the postnatal development of pancreatic islets, evidenced by a gradual inversion of the α-to β-cell ratio. A direct link between diet induced insulin resistance and β-cell dysfunction via disturbed GLUT2 plasma membrane localization has recently been demonstrated. Administration of a high-fat diet feeding in mice results in intracellular retention of the transporter due to improper glycosylation of the protein thereby

The early lethality of GLUT2 deficient mice shortly after birth hindered the analysis of GLUT2 physiology in the different tissues of its expression. Transgenic mice that overexpress GLUT1 specifically in β-cells under the control of the rat insulin promoter were generated (RIPGLUT1/ GLUT2-/-) to study the functional consequences of GLUT2 deficiency in tissues such as liver,

In the liver, GLUT2 deficiency was expected to dramatically affect hepatic glucose output under fasting conditions. Interestingly, hepatic glucose output and glucagon response of the livers from mice lacking GLUT2 were normal. No counterregulation of other transporters known at that time was observed (GLUT1, 3, 4, 5 and SGLT1). Glucose output in GLUT2 deficient livers was not inhibited by cytochalasin B. An alternative membrane traffic-based pathway was proposed that releases glucose directly from the ER after glycogen breakdown or gluconeogenesis. The exact nature of this route has not been determined. In humans GLUT2 deficiency is associated with a marked hypoglycemia in the fasting state owing to a diminished hepatic glucose output and a failure of glucagon to increase plasma glucose. However, human patients with FBS do not generally develop diabetes and do not display a complete loss of their beta cell function, indicating that functionality of human pancreatic β-cells does not solely

Deletion of GLUT2 specifically in the liver [31] or the central nervous system [32] showed, that GLUT2 mediated glucose-sensing mechanisms in both organs exist which control pancreatic

In the kidney, a role for GLUT2 in basolateral sugar reabsorption by tubular epithelial cells was postulated due the glucosuria that has been observed in RIPGLUT1/GLUT2-/-mice. GLUT2 complements the active sugar uptake at the apical epithelium mediated by SGLT2. The functional relevance for GLUT2 in the kidney in humans is supported by the observation of

GLUT3 was cloned from a human fetal muscle cDNA library. GLUT3 is considered as a neuronspecific glucose transporter due to its dominant expression in the brain in various species. However, besides the brain GLUT3 is also expressed in tissues with high demand for glucose such as testes (spermatozoa), placenta, preimplantation embryos or certain cancer cells and cancer tissues. Its tissue distribution and function correspond with its high affinity (Km=1.4 mM for 2-DG), and transport capacity for glucose. While galactose (Km=8.5 mM), mannose,

leading to compromised glucose stimulated insulin secretion.

impaired kidney glucose reabsorption in patients with FBS.

kidney, intestine, and also in the brain.

depend on GLUT2.

22 Glucose Homeostasis

β cell mass and function.

**4.3. GLUT3** *(SLC2A3)*

## *Single nucleotide polymorphisms (SNP's) in the GLUT3 gene have been identified and linked to dyslexia*

Dyslexia is one of the most common learning disorders in school-aged children. Dyslexic children show differences in event-related potential measurements, in particular for mismatch negativity (MMN), which reflects automatic speech deviance processing. Whole-genome association analysis in 200 dyslexic children, focusing on MMN measurements, identified two SNPs that both showed a significant association with mRNA-expression levels of *SLC2A3* on chromosome 12. It was suggested that a possible trans-regulation effect on *SLC2A3* might lead to glucose deficits in dyslexic children that might cause their attenuated MMN in passive listening tasks.

## *Mouse Models of GLUT3 deficiency*

During mouse preimplantation development GLUT3 is expressed at the apical membrane of the trophectoderm layer of the blastocyst and mediates glucose uptake by the embryo from the external (maternal) environment. Knockdown of the transporter by antisense-RNA at this time-point of development disrupts blastocyst development by diminishing uptake of glucose by the embryo. These data indicated a crucial role for GLUT3 during preimplantation embryo development and its deficiency in mice was assumed to result in embryonic lethality before implantation. Indeed, homozygous loss of GLUT3 leads to a complete loss of embryos at day 12.5. However, morulae develop normally to the blastocyst stage and implantation is not affected by loss of GLUT3.

Heterozygous GLUT3 knockout mice have been characterized especially for a potential neuron (brain) specific phenotype. These animals exhibit significantly enhanced cerebrocortical activity and are slightly more sensitive to an acoustic startle stimulus. However, behavior of these animals regarding coordination, reflexes, motor abilities, anxiety, learning, and memory is normal.

Zhao et al. described features of autism spectrum disorders in heterozygous GLUT3 knockout animals as being abnormal spatial learning, working memory, electroencephalographic seizures, and perturbed social behavior with reduced vocalization and stereotypes at low frequency.

## **4.4. GLUT4** *(SLC2A4)*

Besides GLUT1, GLUT4 represents one of the most intensively studied glucose transporters which is attributed to its important physiological role regulating the rate-limiting step in insulin-stimulated glucose uptake of skeletal and cardiac muscle, brown and white adipose tissue. Thereby, impaired GLUT4 translocation is causally linked to insulin resistance and consequently to the disease condition of non-insulin dependent diabetes mellitus.

GLUT4 was cloned in 1989 by various groups from human, rat, and mouse tissues. GLUT4 displays a similar affinity for glucose as GLUT1 with a Km of ~5 mM, and is also capable of transporting dehydroascorbic acid and glucosamine (Km ~3.9 mM). When expressed in yeast *(S. cerevisae*) rat GLUT4 is inhibited by the classical inhibitors cytochalasin B (IC50=0.2 µM), phloretin (IC50=10 µM) and phlorizin (IC50=140 µM).

fat mass, cardiomegaly, and shortened lifespan, but no diabetes. In contrast, heterozy‐ gous GLUT4-null mice develop hyperglycemia and hyperinsulinemia associated with reduced muscle glucose uptake, hypertension and morphological alterations in heart and liver. Rather unexpectedly, about 50% of heterozygous GLUT4 knockout mice develop diabetes before the age of 6 months, a phenotype that can be reversed by selective

Mammalian Sugar Transporters http://dx.doi.org/10.5772/58325 25

In contrast to conventional GLUT4 knockout animals muscle specific GLUT4 deficient mice have normal body and fat pad weight and a normal lifespan. While skeletal muscle mass is unchanged the heart weight is increased similar to GLUT4 deficient mice and heart specific GLUT4 knockout animals. Basal and especially insulin-or contraction-induced glucose uptake into skeletal muscle is reduced, which is causative for the hyperglycemia, glucose intolerance and insulin resistance seen in those animals. A subset of animals develops diabetes. Surpris‐ ingly, insulin stimulated glucose transport in adipose tissue and insulin induced suppression of hepatic glucose production are also impaired which is assumed to be secondary to the

Adipose tissue selective GLUT4 inactivation, unlike the conventional GLUT4 disruption, does not affect growth, adipose mass or size. Cardiac hypertrophy is not seen in those animals. Fat specific GLUT4 null mice are insulin resistant and glucose intolerant and a subset of animals develop diabetes, which is also observed in muscle specific GLUT4 deficient mice. Muscle and liver are insulin-resistant in those animals. Surprisingly, insulin resistance in muscle was only observed in vivo but not ex vivo indicating a systemic impact of adipose tissue on insulin

GLUT4 knockout mice specifically muscle and cardiac tissue specific GLUT4 deficient mice all develop cardiac hypertrophy. While homo-and heterozygous GLUT4 null mice show cardiac dysfunction under normal conditions, cardiac specific GLUT4 knockouts have normal contractile function under basal conditions but decreased recovery after hypoxia. Metabol‐

Searching the human genome for additional genes coding for glucose transporters, a putative gene was identified and cloned showing 95% identity to GLUT3 on the nucleotide level. The *SLC2A14* gene maps to chromosome 12p13.3 with a 10Mb distance to GLUT3 and appears to be a consequence of a gene duplication of GLUT3. GLUT14 was shown to be specifically expressed in the testes. Two alternatively spliced forms of GLUT14 were identified. Interest‐ ingly, GLUT14 has no orthologue in mice, a finding that has also been made for GLUT11.

ically, heart specific GLUT4 knockout mice are normal and have a normal lifespan.

overexpression of GLUT4 in skeletal muscle.

*Muscle-specific GLUT4 knockout mice*

hyperglycemia in those animals.

sensitivity.

**4.6. GLUT14** *(SLC2A14)*

*Adipose tissue-specific GLUT4 knockout mice*

*Cardiac tissue specific GLUT4 knockout mice*
