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The Na+-dependent glucose cotransporters (SGLT) are a family of glucose transporters that mediate an active, sodium-linked transport process against an electrochemical gradient. The SGLT are known to play important roles in absorption of dietary D-glucose and D-galactose from the intestinal lumen and in the reabsorption of D-glucose from the glomerular filtrate in kidney. To study the role and regulation of SGLT in tissues of lactating cows, we cloned and sequenced the full-length cDNA of bovine SGLT1 and SGLT5. Open reading frame analysis predicted that bovine SGLT1 is composed of 664 amino acids with a molecular weight of approximately 73 kDa, and SGLT5 is composed of 597 amino acids with a molecular weight of approximately 65 kDa. The deduced amino acid sequence of bovine SGLT1 is 48% identical and 66% conserved relative to that of bSGLT5. The amino acid sequence of bovine SGLT1 is 97, 88, 87, 86, 85, and 84% identical to sheep, mouse, rat, horse, human, and rabbit SGLT1, respectively. In contrast, the amino acid sequence of bSGLT5 is relatively divergent among species, being 85, 64, and 48% identical to rabbit, human, and rat SGLT5, respectively. Bovine SGLT retain the characteristic structural features of SGLT1 proteins described in other species, including membrane-spanning helices and glucose transporter motifs. The major in vitro transcription and translation product of bovine SGLT1 cDNA migrated at an apparent molecular weight of 52 kDa. In the presence of canine microsomal membranes, the translation product increased to 53 kDa, suggesting glycosylation. The SGLT1 mRNA was most abundant in bovine intestine, at intermediate levels in bovine kidney, and at lower levels in bovine mammary gland, liver, and lung. No SGLT1 mRNA was detected in bovine spleen, skeletal muscle, or testes. Expression of SGLT5 mRNA was found predominantly in bovine kidney, only at very low levels in bovine testes, skeletal muscle, and spleen, and was essentially undetectable in bovine mammary gland, liver, lung, and small intestine. Abundance of SGLT1 mRNA in bovine mammary gland increased more than 4-fold during late pregnancy and early lactation. The sequence and expression data reported in this paper lay the groundwork for future studies aimed at unraveling the functional roles of SGLT in supporting milk production and maintaining glucose homeostasis during lactation.

The second member of the Na(+)/glucose cotransporter family, SGLT2, is a low-affinity active glucose transporter. In humans, it is predominantly located on the apical membrane of the S1 and S2 segments of renal proximal convoluted tubules, and, thus, may be mainly responsible for the reabsorption of D-glucose from the glomerular filtrate. By BLAST searching the GenBank database, we found expressed sequence tag sequences of SGLT2 in the cDNA library of bovine mammary tissues, indicating its expression in bovine mammary gland. To facilitate study of the mechanism of glucose reabsorption in bovine kidneys in maintenance of glucose homeostasis of lactating cows and the potential role of SGLT2 in the mammary gland, we cloned bovine SGLT2 and examined the distribution of its mRNA expression in bovine tissues. The full length mRNA of bSGLT2 is 2275 bp, and is predicted to encode a protein of 673 amino acids, with a molecular weight of approximately 73 kDa. The deduced amino acid sequence of bovine SGLT2 is 91, 90, 91, and 90% identical to human, rabbit, mouse, and rat SGLT2, respectively, and is 58 and 48% identical to bovine SGLT1 and SGLT5, respectively. The sequence of bSGLT2 contains several characteristically conserved sodium:solute symporter family signatures. Analysis of current bovine genomic data indicates that the bovine SGLT2 gene may consist of 14 exons. The major in vitro transcription and translation product of bovine SGLT2 cDNA migrated at an apparent molecular weight of 55 kDa. The SGLT2 mRNA was detected predominantly in bovine kidney as a 2.3-kb transcript, and at lower levels in all other bovine tissues examined, including the mammary gland, liver, lung, spleen, intestine, and skeletal muscle, as a 3.0-kb transcript. Expression of SGLT2 mRNA in bovine mammary gland increased more than 10-fold from late pregnancy to early lactation, similar to SGLT1. This indicates that SGLT2 may play a role in milk synthesis in the lactating mammary gland.

Polycomb group (PcG) protein-mediated histone methylation (H3K27me3) controls the correct spatiotemporal expression of numerous developmental regulators in Arabidopsis. Epigenetic silencing of the stem cell factor WUS in floral meristems (FMs) depends on H3K27me3 deposition by PcG proteins. However, the role of H3K27me3 in silencing of other meristematic regulator and pluripotency genes during FM determinacy has not yet been studied. To this end, we report the genome-wide dynamics of H3K27me3 levels during FM arrest and the consequences of strongly depleted PcG activity on early flower morphogenesis including enlarged and indeterminate FMs. Strong depletion of H3K27me3 levels results in misexpression of the FM identity gene AGL24, which partially leads to floral reversion causing ap1-like flowers and indeterminate FMs expressing ectopically WUS and STM. Loss of STM can rescue supernumerary floral organs and FM indeterminacy in H3K27me3-deficient flowers indicating that the hyperactivity of the FMs is at least partially a result of ectopic STM expression. Nonetheless, WUS remained essential for the FM activity. Our results demonstrate that PcG proteins promote FM determinacy at multi-levels of the floral gene regulatory network, silencing initially floral regulators like AGL24 that promotes FM indeterminacy, and subsequently, meristematic pluripotency genes such as WUS and STM during FM arrest.