Molecular insights into the regulation of glutamine transport across cellular membranes
Creators: name: Balkrishna, Sarojini
Publisher: name: The Australian National University
Year: 2013
Subjects: subject: QP562.G55 B35 2013
subject: Glutamine Physiological transport
subject: Messenger RNA
subject: Carrier proteins
subject: Genetic regulation

description: Glutamine is the most abundant amino acid in the body. As the primary nitrogen carrier between cells, it plays essential roles in inter-organ nutrition and cellular homeostasis. The transport of glutamine across cellular membranes is facilitated by membrane transporters of the Solute-Linked Carrier (SLC) families. SLC38 is one such family, whose members encode SNAT proteins (Sodium-Neutral Amino acid Transporter or System N/A Transporter), which together transport a broad range of neutral amino acids. Encoded by S/c38a3, SNAT3 prefers glutamine, histidine and asparagine as substrates. Substrate transport can be bidirectional since it is accompanied by Na{u207A} symport and H{u207A} antiport. S/c38a3 has a narrow tissue specificity, and is expressed in the central nervous system, liver, kidney and pancreas. In the central nervous system, glutamine efflux via SNAT3 in astrocytes facilitates neurotransmitter recycling to maintain effective neurotransmission and prevent neurotoxicity. In the liver, glutamine uptake and its subsequent breakdown in hepatocytes facilitates in the detoxification of ammonia by feeding into the urea cycle. In the kidney, as the sole portal for glutamine entry through the basolateral membranes of proximal tubules, SNAT3 is required to maintain acid-base balance, especially during metabolic acidosis. In the pancreas, SNAT3 is thought to facilitate insulin release. Despite being at the start of such key metabolic processes, the molecular mechanisms governing the regulation of SNAT3 is a lesser explored area. This thesis examined the regulation of SNAT3 at an epigenetic, transcriptional, post{u00AD} transcriptional and post-translational level. Firstly, this thesis showed that glutamine uptake via SNAT3 was down-regulated by the Protein Kinase C (PKC). Using Xenopus laevis oocytes, it was shown that the down-regulation occurred in a caveolin-dependent, dynamin-independent manner. Furthermore, the effect of PKCon SNAT3 activity was only observed in hepatocytes, and not in astrocytes. Secondly, in an attempt to understand the causes for the narrow tissue specificity of the gene, the transcriptional regulation of S/c38a3 was studied. Analyses of the S/c38a3 genomic sequence identified the position of the gene core promoter merely 5Obp upstream of the transcriptional start site. Site-directed mutagenesis and chromatin immunoprecipitation demonstrated that transcription was driven by the ubiquitous transcription factor, Specificity Protein 1 (Spl). Bisulfite genome sequencing revealed that four cytosine residues in the proximal promoter region were methylated in the non-S/c38a3 expressing mouse intestine and unmethylated in the mouse liver. Finally, the mechanisms governing the upregulation of S/c38a3 mRNA during metabolic acidosis were examined. The results from this study demonstrated that there may be transcriptional upregulation of the gene during acute acidosis. Additionally, the S/c38a3 3' untranslated region contained elements that destabilized the mRNA during normal conditions, and stabilized it during chronic acidosis. Moreover, these results demonstrated that S/c38a3 was a part of a common regulatory pathway that controlled multiple genes that are upregulated during metabolic acidosis. Taken together, this thesis has shed light to the regulatory events that control glutamine transport via SNAT3 at a molecular level.