Ursula Martiné

Partial deficiency of Thyroid transcription factor 1 produces predominantly neurological defects in humans and mice

Three genes, TTF1, TTF2, and PAX8, involved in thyroid gland development and migration have been identified. Yet systematic screening for defects in these genes in thyroid dysgenesis gave essentially negative results. In particular, no TTF1 gene defects were found in 76 individuals with thyroid dysgenesis even though a deletion of this gene in the mouse results in thyroid and lung agenesis and defective diencephalon. We report a 6-year-old boy with predominant dyskinesia, neonatal respiratory distress, and mild hyperthyrotropinemia. One allele of his TTF1 gene had a guanidine inserted into codon 86 producing a nonsense protein of 407, rather than 371, amino acids. The mutant TTF1 did not bind to its canonical cis-element or transactivate a reporter gene driven by the thyroglobulin promoter, a natural target of TTF1. Failure of the mutant TTF1 to interfere with binding and transactivation functions of the wild-type TTF1 suggested that the syndrome was caused by haploinsufficiency. This was confirmed in mice heterozygous for Ttf1 gene deletion, heretofore considered to be normal. Compared with wild-type littermates, Ttf1(+/-) mice had poor coordination and a significant elevation of serum thyrotropin. Therefore, haploinsufficiency of the TTF1 gene results in a predominantly neurological phenotype and secondary hyperthyrotropinemia.

Role of Neutral Amino Acid Transport and Protein Breakdown for Substrate Supply of Nitric Oxide Synthase in Human Endothelial Cells

Abstract— Endothelial dysfunction is often associated with a relative substrate deficiency of the endothelial nitric oxide synthase (eNOS) in spite of apparently high intracellular arginine concentrations. For a better understanding of the underlying pathophysiological mechanisms, we aimed to characterize the intracellular arginine sources of eNOS. Our previous studies in human endothelial EA.hy926 cells suggested the existence of two arginine pools: pool I can be depleted by extracellular lysine, whereas pool II is not freely exchangeable with the extracellular space, but accessible to eNOS. In this study, we demonstrate that the eNOS accessible pool II is also present in human umbilical vein endothelial cells (HUVECs), but not in ECV bladder carcinoma cells transfected with an expression plasmid for eNOS. In the endothelial cells, one part of pool II (referred to as pool IIA) consisted of recycling of citrulline to arginine. This part could be depleted by neutral amino acids that match the substrate profile of system N transporter 1 (SN1), presumably by the removal of intracellular citrulline. SN1 was expressed in EA.hy926 cells and HUVECs as shown by real-time RT-PCR. The second part of pool II (referred to as pool IIB) could not be depleted by any of the cationic or neutral amino acids tested. Our data demonstrate that pool IIB is nourished by protein breakdown and thus represents a substrate pool likely to accumulate protein-derived endogenous inhibitors of eNOS. Preferential use of the arginine pool IIB under pathophysiological conditions might therefore explain the arginine paradox.

Expression of solute carrier 7A4 (SLC7A4) in the plasma membrane is not sufficient to mediate amino acid transport activity

Member 4 of human solute carrier family 7 (SLC7A4) exhibits significant sequence homology with the SLC7 subfamily of human cationic amino acid transporters (hCATs) [Sperandeo, Borsani, Incerti, Zollo, Rossi, Zuffardi, Castaldo, Taglialatela, Andria and Sebastio (1998) Genomics 49, 230-236]. It is therefore often referred to as hCAT-4 even though no convincing transport activity has been shown for this protein. We expressed SLC7A4 in Xenopus laevis oocytes, but could not detect any transport activity for cationic, neutral or anionic amino acids or for the polyamine putrescine. In addition, human glioblastoma cells stably overexpressing a fusion protein between SLC7A4 and the enhanced green fluorescent protein (EGFP) did not exhibit an increased transport activity for l-arginine. The lack of transport activity was not due to a lack of SLC7A4 protein expression in the plasma membrane, as in both cell types SLC7A4-EGFP exhibited a similar subcellular localization and level of protein expression as functional hCAT-EGFP proteins. The expression of SLC7A4 can be induced in NT2 teratocarcinoma cells by treatment with retinoic acid. However, also for this endogenously expressed SLC7A4, we could not detect any transport activity for l-arginine. Our data demonstrate that the expression of SLC7A4 in the plasma membrane is not sufficient to induce an amino acid transport activity in X. laevis oocytes or human cells. Therefore, SLC7A4 is either not an amino acid transporter or it needs additional (protein) factor(s) to be functional.

LONG-TERM PERSISTENCE OF HEPATITIS B VIRUS DNA IN THE SERUM OF CHILDREN WITH CHRONIC HEPATITIS B AFTER HEPATITIS B E ANTIGEN TO ANTIBODY SEROCONVERSION

To evaluate the prevalence and duration of viremia in relation to the features of liver disease, we investigated hepatitis B virus (HBV) DNA by the polymerase chain reaction in the serum of 39 children with chronic hepatitis B, after hepatitis B e antigen to antibody seroconversion. During a mean observation period of 8.2 +/- 3.8 years after seroconversion, all patients were asymptomatic; 36 had persistently normal alanine aminotransferase levels, and three had occasional mild alterations. Liver histology, checked in 21 patients, showed persistent hepatitis in nine, fibrosis in 10, and cirrhosis in two cases. HBV DNA was always undetectable by dot blot hybridization. Five children eventually cleared hepatitis B surface antigen, including one with cirrhosis who developed liver cancer at 19 years. HBV DNA was detected by polymerase chain reaction in 87% of children within 5 years of follow-up, in 58% of cases 6-10 years after seroconversion (p < 0.001), and in 50% of patients investigated later. Long-term viremia was found in two patients (40%) who cleared HBsAg, including the one who developed liver cancer. The chances of clearing viremia during follow-up were higher in children with acute hepatitis at the onset of illness (86%) than in those with asymptomatic onset (37%; p < 0.05). Our results show that low levels of HBV viremia, probably reflecting low levels of virus replication, persist for several years in children with chronic hepatitis B after hepatitis B e antigen to antibody seroconversion and remission of liver disease, even after the clearance of hepatitis B surface antigen. Persistent replication could support mild biochemical alterations and inflammatory liver lesions. It could allow late reactivation of liver disease and may play a role in the development of carcinoma.

Activation of classical protein kinase C reduces the expression of human cationic amino acid transporter 3 (hCAT-3) in the plasma membrane

We have previously shown that activation of PKC (protein kinase C) results in internalization of hCAT-1 [human CAT-1 (cationic amino acid transporter 1)] and a decrease in arginine transport [Rotmann, Strand, Martiné and Closs (2004) J. Biol. Chem. 279, 54185-54192]. However, others found increased transport rates for arginine in response to PKC activation, suggesting a differential effect of PKC on different CAT isoforms. Therefore we investigated the effect of PKC on hCAT-3, an isoform expressed in thymus, brain, ovary, uterus and mammary gland. In Xenopus laevis oocytes and human U373MG glioblastoma cells, hCAT-3-mediated L-arginine transport was significantly reduced upon treatment with compounds that activate classical PKC. In contrast, inactive phorbol esters and an activator of novel PKC isoforms had no effect. PKC inhibitors (including the PKCalpha-preferring Ro 31-8280) reduced the inhibitory effect of the PKC-activating compounds. Microscopic analyses revealed a PMA-induced reduction in the cell-surface expression of fusion proteins between hCAT-3 and enhanced green fluorescent protein expressed in X. laevis oocytes and glioblastoma cells. Western-blot analysis of biotinylated surface proteins demonstrated a PMA-induced decrease in hCAT-3 in the plasma membrane, but not in total protein lysates. Pretreatment with a PKC inhibitor also reduced this PMA effect. It is concluded that similar to hCAT-1, hCAT-3 activity is decreased by PKC via reduction of transporter molecules in the plasma membrane. Classical PKC isoforms seem to be responsible for this effect.

Relative contribution of different l-arginine sources to the substrate supply of endothelial nitric oxide synthase

In certain cases of endothelial dysfunction l-arginine becomes rate-limiting for NO synthesis in spite of sufficiently high plasma concentrations of the amino acid. To better understand this phenomenon, we investigated routes of substrate supply to endothelial nitric oxide synthase (eNOS). Our previous data with human umbilical vein (HUVEC) and EA.hy.926 endothelial cells demonstrated that eNOS can obtain its substrate from the conversion of l-citrulline to l-arginine and from protein breakdown. In the present study, we determined the quantitative contribution of proteasomal and lysosomal protein degradation and investigated to what extent extracellular peptides and l-citrulline can provide substrate to eNOS. The RFL-6 reporter cell assay was used to measure eNOS activity in human EA.hy926 endothelial cells. Individual proteasome and lysosome inhibition reduced eNOS activity in EA.hy926 cells only slightly. However, the combined inhibition had a pronounced reducing effect. eNOS activity was fully restored by supplementing either l-citrulline or l-arginine-containing dipeptides. Histidine prevented the restoration of eNOS activity by the dipeptide, suggesting that a transporter accepting both, peptides and histidine, mediates the uptake of the extracellular peptide. In fact, the peptide and histidine transporter PHT1 was expressed in EA.hy926 cells and HUVECs (qRT/PCR). Our study thus demonstrates that l-citrulline and l-arginine-containing peptides derived from either intracellular protein breakdown or from the extracellular space seem to be good substrate sources for eNOS.