Cell Glucose Transport and Glucose Handling During Fetal and Neonatal Development

Introduction

Glucose is the primary substrate for the growing and developing fetus, and in normal human pregnancies there is little fetal gluconeogenesis. Glucose is required by most cells for oxidative and nonoxidative adenosine triphosphate (ATP) production and serves as a precursor for other carbon-containing compounds. It is the primary fuel used for several specialized cells and is the major fuel used by the brain. Its storage in the liver as glycogen provides a means by which glucose homeostasis can be maintained, particularly during the neonatal period. Glycogen stores also represent the primary source of energy for muscle tissue during exercise in postnatal life. Because of the diverse metabolic roles played by glucose, defects in its uptake or metabolism can alter cellular functions and can lead to significant morbidity and mortality. This chapter focuses on the molecular biology and regulation of glucose transporters (GLUTs) in the fetus and newborn.

The plasma membranes of most mammalian cells, except those of the proximal kidney and small intestine, have a passively mediated transport system for glucose. Facilitative entry of glucose into the cell is controlled by GLUTs, structurally related proteins that are encoded by a gene family and are expressed in a tissue-specific manner. A different family of proteins, sodium (Na ⁺ )-coupled transporters (SLGTs), actively transport glucose across the apical membranes of polarized intestinal and renal epithelial cells. The driving force for active glucose absorption is the electrochemical Na ⁺ gradient across the membrane.

Most cells contain at least one GLUT isoform, and many contain more than one ( Table 39.1 ). Furthermore, there are changes in distribution during development ( Table 39.2 ). In most cell types, GLUTs mediate a net uptake of glucose. Under some circumstances, glucose is transported out of the cell. For example, the Na ⁺ -coupled transporter actively transports glucose into epithelial cells of the small intestine, and a facilitative transporter mediates the efflux of glucose from the cell into the interstitium. In hepatocytes, facilitative GLUTs are responsible for the uptake of glucose from the portal circulation and for the release of glucose generated by glycogenolysis or gluconeogenesis. Thus GLUTs ensure efficient tissue uptake and distribution of glucose.

Sodium-Dependent Glucose Transporters

It has long been known that dietary sugars are actively absorbed from the small intestine; however, only recently has the molecular mechanism been elucidated. Active absorption of glucose across epithelial cells of the small intestine and the kidney proximal tubule is accomplished by Na ⁺ -glucose cotransporters located in the brush border membranes. Transport of each glucose molecule is coupled to the cotransport of two Na ⁺ ions (SGLT1) or of one Na ⁺ ion (SGLT2). This transport system uses the energy from an extracellular to intracellular Na ⁺ ion electrochemical gradient, generated by Na ⁺ ,K ⁺ -ATPases, to drive the accumulation of glucose into the cell. These transporters belong to a major class of membrane proteins called cotransporters (or symporters ) and exist in bacteria, plants, and animal membranes, and they actively transport sugars, amino acids, carboxylic acids, and some ions (chloride, phosphate, sulfate, iodide) into cells.

SGLT1 is a hydrophobic integral membrane protein with approximately 12 membrane-spanning domains. The gene encoding the human intestinal SGLT has been localized to the q11.2–qter region of chromosome 22. SGLT1 is a high-affinity, low-capacity transporter protein and is abundantly expressed in the brush border of the small intestine and at lower levels in kidney, lung, heart, pancreas, eyes, tongue, prostate, uterus, salivary glands, and liver.

Clinical interest in the intestinal brush border Na ⁺ -glucose cotransporter has focused on diarrhea and malabsorption. Glucose-galactose malabsorption is a rare autosomal recessive disorder characterized by onset of severe, watery diarrhea in the newborn period. Unless glucose and galactose are eliminated from the diet, death rapidly ensues. Wright and colleagues demonstrated that a single missense mutation in the gene encoding the intestinal Na ⁺ -glucose cotransporter is sufficient to cause life-threatening diarrhea.

SGLT2 complementary DNA (cDNA) was originally isolated by Hediger and colleagues from a human cDNA library. The SGLT amino acid sequences are approximately 60% identical to those of SGLT1, and the proteins have the same predicted secondary structure. The expression of this cotransporter is restricted to the renal cortex and is located in epithelial cells of proximal tubule S1 segments. It is generally thought that the bulk of the filtered glucose is reabsorbed in the proximal convoluted tubule by a low-affinity, high-capacity SGLT2 and that the remainder is reabsorbed by the high-affinity cotransporter SGLT1.

Familial renal glycosuria is an autosomal dominant disorder (an autosomal recessive mode of inheritance has not been excluded in all cases) affecting 0.2% to 0.6% of the general population and is characterized by the excretion of large amounts of glucose into the urine in the presence of normal blood glucose concentrations. The molecular basis of benign renal glycosuria has not been determined. It is possible that mutations in the low-affinity Na ⁺ -glucose cotransporter SGLT2 may be responsible for the defect in renal absorption of filtered glucose.

Na ⁺ -glucose cotransporters appear to be active prenatally, and, as a consequence, the intestine is ready to absorb the first ingested glucose. The cloned cDNAs and specific antibodies for the different Na ⁺ -glucose cotransporters will be valuable tools for identifying the specific cells in the intestine and kidney that express these proteins and for studying the regulation of their expression during development and in altered metabolic states such as diabetes mellitus or pregnancy.

There has been a great deal of interest in the development of novel agents to inhibit SGLT2 as a means to control glucose levels and augment calorie-wasting leading to weight loss in adults with type 2 diabetes. However, to date, there are no studies on the safety and efficacy of this class of drugs in pregnancy.

Facilitated Glucose Transporters

The energy-independent process of transporting glucose across the cell membrane occurs by facilitative diffusion. Transport of glucose is saturable, stereoselective, and bidirectional. The kinetics of glucose transport inward and outward are not necessarily identical, and, in fact, in the erythrocyte, the rate of exchange flux for glucose is faster than net flux. The primary function of the facilitative GLUTs is to mediate the exchange of glucose between blood and the cytoplasm of the cell. This may involve a net uptake or output of glucose from the cell, depending on the type of cell in question, its metabolic state, and the metabolic state of the organisms. In most cells, cytoplasmic glucose is rapidly phosphorylated by hexokinase or glucokinase, levels of glucose-6-phosphatase are low, and therefore there is little intracellular free glucose. These cells are involved only in net uptake and metabolism of blood glucose. The hepatocyte is also a net producer of glucose in the postabsorptive state. Glycogenolysis and gluconeogenesis increase intracellular free glucose to levels greater than its concentration in the blood and result in net efflux of glucose from the cell. In the postprandial state, glucose is transported into the hepatocyte to replenish glycogen stores.

The facilitative GLUTs comprise a family of structurally related proteins. Six facilitated GLUT isoforms have been identified and are designated GLUT, and the gene name is SLC2A. ^{, ,} Several additional GLUTs (see Table 39.1 ) have been identified. Isoforms are expressed in a tissue-specific manner, reflecting the unique glucose requirements of various tissues.

These proteins vary in size from 492 to 524 amino acids. They exhibit 39% to 68% sequence identity and 50% to 76% sequence similarity in pairwise comparisons. ^{, , , ,} A topology map of the GLUTs has been proposed based on analysis of the primary amino acid sequence of GLUT1. Each isoform consists of 12 membrane-spanning domains, an intracytoplasmic hydrophilic loop, and an exofacial loop bearing a single N- glycosylation site. Both the amino and carboxy terminals are exposed intracellularly ( Fig. 39.1 ). Comparisons among the different isoforms have revealed that the sequences of the transmembrane segments and the short cytoplasmic loops connecting these transmembrane regions are highly conserved. Most likely, these regions are responsible for the transport of glucose. The NH ₂ and COOH-terminals are unique for each of the different isoforms and may contribute to isoform-specific properties, such as kinetics, hormone sensitivity, and subcellular localization. ^{, , , ,}

Structure and Properties of Facilitative Glucose Transporters