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GLUT8 Is a Glucose Transporter Responsible for Insulin-Stimulated Glucose Uptake in the Blastocyst

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GLUT8 Is a Glucose Transporter Responsible for Insulin-Stimulated Glucose Uptake in the Blastocyst GLUT8 is a glucose transporter responsible for insulin-stimulated glucose uptake in the blastocyst Mary O. Carayannopoulos*, Maggie M.-Y. Chi*, Ying Cui*, Joyce M. Pingsterhaus*, Robert A. McKnight†, Mike Mueckler‡, Sherin U. Devaskar†, and Kelle H. Moley*‡§ *Department of Obstetrics and Gynecology, 4911 Barnes-Jewish Hospital Plaza, St. Louis, MO 63110; †Department of Pediatrics, University of California, 10833 LeConte Avenue, B-375 MDCC, Los Angeles, CA 90095-1752; and ‡Department of Cell Biology and Physiology, Washington University School of Medicine, 660 South Euclid, St. Louis, MO 63110 Communicated by David M. Kipnis, Washington University School of Medicine, St. Louis, MO, April 14, 2000 (received for review February 16, 2000) Mammalian preimplantation blastocysts exhibit insulin-stimulated using the TBLASTX program. Two rodent clones, AA052119 glucose uptake despite the absence of the only known insulin- and H34451, were identified, and they encoded the C terminus regulated transporter, GLUT4. We describe a previously unidenti- and 3Ј untranslated regions of a previously uncharacterized fied member of the mammalian facilitative GLUT superfamily that GLUT. These clones were used to screen a mouse blastocyst exhibits Ϸ20–25% identity with other murine facilitative GLUTs. cDNA library obtained from Barbara Knowles (The Jackson Insulin induces a change in the intracellular localization of this Laboratory). This plasmid library was made from poly(A) protein, which translates into increased glucose uptake into the RNA from a pool of day-5 mouse blastocysts as described (10). blastocyst, a process that is inhibited by antisense oligoprobes. The original library was altered by cloning it into pSport Presence of this transporter may be necessary for successful blas- (GIBCO) by using SalI and NotI sites (cloning site changes tocyst development, fuel metabolism, and subsequent implanta- made by Sue-Yun Hwang, The Jackson Laboratory). A partial tion. Moreover, the existence of an alternative transporter may cDNA clone was identified, and gene-specific primers were explain examples in other tissues of insulin-regulated glucose used to identify the remainder of the GLUT8 cDNA by using transport in the absence of GLUT4. 5Ј rapid amplification of cDNA ends (CLONTECH) with RNA obtained from the murine embryonic teratocarcinoma lucose transport activity in early preimplantation murine cell line F9. A full-length cDNA was PCR amplified from F9 Gembryos has been attributed to the known facilitative cDNA, and the amplified product was sequenced in both glucose transporters, GLUT1, GLUT2, and GLUT3 (1–4). directions. GLUT1 is present throughout the preimplantation period, which begins with the one-cell embryo and ends with the blastocyst GLUT8 mRNA Analysis. Total RNA was isolated from embryonic- ͞ stage. GLUT2 and GLUT3 are first expressed at a late eight-cell day-18 and 8-week-old BALB c mouse tissues according to the ␮ stage and remain present for the rest of the preimplantation method of Chomczynski and Sacchi (11). Total RNA (15 g) was period. The simultaneous appearance of all three transporters electrophoresed overnight, transferred to a GenScreen Plus corresponds to the critical time in mammalian development membrane, and crosslinked. A 163-bp fragment of GLUT8 32 when embryonic fuel metabolism switches from the oxidation of cDNA was amplified, diluted 1:100, reamplified, and P-labeled lactate and pyruvate via the Krebs cycle and oxidative phos- by using the Klenow fragment (Ambion, Austin, TX). phorylation to anaerobic metabolism of glucose via glycolysis. The blot containing the mRNA was prehybridized for2hat This critical substrate change is thought to be due to the 42°C and subsequently hybridized overnight at 42°C in the same ϫ 6 ͞ CELL BIOLOGY biosynthetic and developmental demands placed on the embryo solution containing 1.68 10 cpm ml. The blot was washed two ϫ ͞ as the blastocyst creates the fluid-filled blastocoel and prepares times at room temperature in 2 SSC 0.5% SDS and then for implantation. We have also shown that a significant decrease washed three times for 10 min each at 65°C. The signal on the in glucose transport at this stage of development induced by blot was detected in a Molecular Analyst PhosphorImager. The maternal diabetes occurs concurrently with a premature and blot was stripped and reprobed with an S2 ribosomal probe as an exaggerated increase in apoptosis in the early embryo (5). This internal control (12). occurrence suggests that optimal glucose transport mechanisms are necessary for preimplantation embryo survival. GLUT8 Protein Analysis. A polyclonal antibody was raised against Both the insulin receptor and insulin-like growth factor-1 a keyhole limpet conjugate to the last 11 amino acids of the (IGF-1) receptor are expressed at approximately the same C-terminal tail (LEQVTAHFEGR) of GLUT8 by immunization time as this increase in glucose demand (6, 7). IGF-1, a ligand of rabbits (Charles River Pharmaceuticals, Southbridge, MA). for both receptors, is present from the two-cell stage onward, For overexpression studies, GLUT8 tagged with FLAG at the N whereas insulin is not made by the embryo (8). Others have terminus was transiently transfected by diethylaminoethyl- shown that insulin and IGF-1 regulate glucose transport in dextran in COS-7 cells. The cell lysates were solubilized in SDS, ␮ mouse blastocysts via the IGF-1 receptor (9). The only known and 400 g of total protein was subjected to immunoprecipita- ␮ insulin-regulated transporter, GLUT4, however, is not ex- tion with 10 g of anti-FLAG M2 monoclonal antibody (Sigma). pressed at any time during preimplantation development (2). For these reasons, we hypothesized that a previously uniden- Abbreviations: GLUT, glucose transporter; IGF, insulin-like growth factor; TM, transmem- tified, insulin-regulated GLUT may be expressed at this stage brane region; ICM, inner cell mass. of development. Data deposition: The sequence reported in this paper has been deposited in the GenBank database (accession no. AF232061). Methods §To whom reprint requests should be addressed. E-mail: [email protected]. Cloning and Sequence Comparison of GLUT8. A homologous amino The publication costs of this article were defrayed in part by page charge payment. This acid sequence from the C terminus of GLUT1–5 was used to article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. search the National Center for Biotechnical Information by §1734 solely to indicate this fact. PNAS ͉ June 20, 2000 ͉ vol. 97 ͉ no. 13 ͉ 7313–7318 Downloaded by guest on October 2, 2021 The immunoprecipitation complexes were subjected to SDS͞ PAGE and transferred to nitrocellulose. The blots were immu- noblotted with GLUT8 antiserum (1:500), GLUT8 antiserum (1:500) plus 50 molar excess peptide, or anti-FLAG M2 (1 ␮g͞ml), and enhanced chemiluminescence was used to detect the proteins. For the tissue Western blots, 100 ␮g of total protein was added to sample buffer, subjected to SDS͞PAGE, and trans- ferred to nitrocellulose. For the embryo Western blot, 279 blastocysts were pooled, added to 5ϫ sample buffer, subjected to SDS͞PAGE, and transferred. Immunoblotting was done as described above. GLUT8 Localization Within the Mouse Blastocyst. Two-cell and four- ϫ cell embryos were obtained from superovulated (B6 SJL)F1 (The Jackson Laboratory) female mice 48 h after mating and cultured to a blastocyst stage as described (1). The blastocysts were fixed on glass slides and immunostained with GLUT8 peptide-purified antiserum (10 ␮g͞ml). Fluorescence was de- tected with laser-scanning confocal immunofluorescent micros- copy (Bio-Rad MRC-600) as described (1). As controls, fixed blastocysts were stained with preimmune serum or with immune serum plus a 50-fold molar excess of the peptide used to raise the antibody. Fig. 1. GLUT8 sequence analysis. Protein alignment of mouse GLUT8 with murine GLUT1–4. The alignment was made with MEGALIGN in the DNASTAR Functional Characterization of GLUT8 in the Mouse Blastocyst. Anti- program with the clustal method. Gray shading indicates positions that are sense and hyperglycemia mouse embryo experiments. Two-cell identical in all transporters; open boxes identify residues shared by four of five embryos were obtained as described above and cultured to a transporters. The approximate positions of the transmembrane regions are blastocyst stage under one of the following conditions: (i)5␮M denoted by a bar and the abbreviation TM. GLUT8 sense oligoprobe (5Ј-ATGAGTCCCGAGGAC- Ј ii ␮ Ј CCCCAG-3 ), ( )5 M GLUT8 antisense oligoprobe (5 - immunofluorescent microscopy studies, at least 12 embryos were CTGGGGGTCCTCGGGACTCAT-3Ј), or (iii) medium with examined for each experimental group on three different days. nothing added. Nonradioactive insulin-stimulated 2-deoxyglu- For the oocyte glucose uptake studies, at least 12 oocytes were cose uptake into single blastocysts was performed with microflu- assayed in two different experiments. Significance was defined as orometric assays combined with enzymatic cycling reactions as P Ͻ 0.01. described (1) with the following modifications. Blastocysts from each condition were incubated in medium at a final glucose Results concentration of 5.6 mM with 500 nM insulin (Sigma, bovine Cloning and Structure of GLUT8. A full-length 1,843-bp cDNA pancreas) for 30 min, and 2-deoxyglucose uptake was then measured as described (1). For the high-glucose conditions, encoding a 478-amino acid protein was cloned from the
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