The proton/amino acid cotransporter PAT2 is expressed in neurons with a different subcellular localization than its paralog PAT1

The new member of the mammalian amino acid/auxin permease family, PAT2, has been cloned recently and represents an electrogenic proton/amino acid symporter. PAT2 and its paralog, PAT1/LYAAT-1, are transporters for small amino acids such as glycine, alanine, and proline. Our immunodetection studies revealed that the PAT2 protein is expressed in spinal cord and brain. It is found in neuronal cell bodies in the anterior horn in spinal cord and in brain stem, cerebellum, hippocampus, hypothalamus, rhinencephalon, cerebral cortex, and olfactory bulb in the brain. PAT2 is expressed in neurons positive for the N-methyl-d-aspartate subtype glutamate receptor subunit NR1. PAT2 is not found in lysosomes, unlike its paralog PAT1, but is present in the endoplasmic reticulum and recycling endosomes in neurons. PAT2 has a high external proton affinity causing half-maximal transport activation already at a pH of 8.3, suggesting that its activity is most likely not altered by physiological pH changes. Transport of amino acids by PAT2 activity is dependent on membrane potential and can occur bidirectionally; membrane depolarization causes net glycine outward currents. Our data suggest that PAT2 contributes to neuronal transport and sequestration of amino acids such as glycine, alanine, and/or proline, whereby the transport direction is dependent on the sum of the driving forces such as substrate concentration, pH gradient, and membrane potential.     

Identification of a mammalian angiopoietin-related protein expressed specifically in liver

Based on searches of EST databases for signal sequences and amphipathic helices, we have identified and cloned an angiopoietin-like gene, ANGPTL3. Multiple tissue Northern blots show that ANGPTL3 is expressed principally in the liver. ANGPTL3 is expressed early during liver development, and expression is maintained in adult liver. Human ANGPTL3 is a 460-amino-acid polypeptide with the characteristic structure of angiopoietins: a signal peptide, an extended helical domain predicted to form dimeric or trimeric coiled-coils, a short linker peptide, and a globular fibrinogen homology domain (FHD). Murine ANGPTL3 is a 455-acid polypeptide encoded by seven exons on mouse chromosome 4, spanning about 11 kb of DNA. ANGPTL3 contains the four conserved cysteines implicated in the intramolecular disulfide bonds within the FHD, but it does not contain two other cysteines that are found within the FHD of angiopoietins 1, 2, and 4. ANGPTL3 also does not contain the characteristic calcium binding motif found in the other angiopoietins. By radiation hybrid mapping and the use of surrounding genes, human ANGPTL3 maps to the 1p31 region.     

Ex-FABP,extracellular fatty acid binding protein,is a stress lipocalin expressed during chicken embryo development

Extracellular Fatty Acid Binding Protein (Ex-FABP) is a 21 kDa lipocalin, expressed during chicken embryo development in hypertrophic cartilage, in muscle fibres and in blood granulocyte. The protein selectively binds with high affinity fatty acids, preferably long chain unsaturated fatty acids in chondrocyte and myoblast cultures Ex-FABP expression is increased by inflammatory-agents and repressed by anti-inflammatory-agents. In adult cartilage, Ex-FABP is expressed only in pathological conditions such as in dyschondroplastic and osteoarthritic chicken cartilage. We propose that lipocalin Ex-FABP represents a stress protein physiologically expressed in tissues where active remodelling is taking place during development and also present in tissues characterized by a stress response due to pathological conditions.     

Psg18 is specifically expressed in follicle-associated epithelium

Pregnancy-specific glycoproteins (Psgs) secreted by the placenta regulate the immune system to ensure the survival of the fetal allograft by inducing IL-10, an anti-inflammatory cytokine. However, it is unknown whether Psgs are involved in more general aspects of immune response other than maternal immunity. Here, we report that Psg18 is highly expressed in the follicle-associated epithelium (FAE) overlaying Peyer's patches (PPs). Bioinformatics analysis with Reference Database for Immune Cells (RefDIC) as well as RT-PCR data demonstrated that Psg18 is exclusively expressed in FAE in adult mice, in contrast to other Psg family members that are either not expressed or only slightly expressed in FAE. Psg18 expression was observed in FAE of germ-free-conditioned mice, and was slightly upregulated after bacterial inoculation. In situ hybridization analysis revealed that Psg18 is widely expressed throughout FAE. Furthermore, Psg18 protein is deposited on the extracellular matrix in the subepithelial dome beneath FAE, where antigen-presenting cells accumulate. These results suggest that Psg18 is an FAE-specific marker protein that could promote interplay between FAE and immune cells in mucosa-associated lymphoid tissues.     

Excitatory Amino Acid Transporter 5 is widely expressed in peripheral tissues

It is routinely stated in the literature that Excitatory Amino Acid Transporter 5 (EAAT5) is a retina-specific glutamate transporter. EAAT5 is expressed by retinal photoreceptors and bipolar cells, where it serves as a slow transporter and as an inhibitory glutamate receptor, the latter role is due to the gating of a large chloride conductance. The dogma of an exclusively retinal distribution has arisen because Northern blot analyses have previously shown only modest hybridisation in non-retinal tissues. Others have re-interpreted this as indicating that EAAT5 was only present in retinal tissues. However, this view appears to be erroneous; recent evidence demonstrating abundant expression of EAAT5 in rat testis prompted us to re-examine this dogma. A new antibody was developed to an intracellular loop region of rat EAAT5. This new tool, in concert with RT-PCR and sequencing, demonstrated that EAAT5 is widely distributed at the mRNA and protein levels in many non-nervous tissues including liver, kidney, intestine, heart, lung, and skeletal muscle. We conclude that EAAT5 is a widely distributed protein. Whether it functions in all locations as a glutamate transporter, or mainly as a glutamate-gated chloride conductance, remains to be determined.Key words: EAAT5, glutamate, transporter, heart, lung, kidney.     

The anti-apoptotic protein lifeguard is expressed in breast cancer cells and tissues

Lifeguard (LFG) is an anti-apoptotic protein that inhibits Fas-mediated death in tumour cells. However, the molecular function of human LFG in the carcinogenesis of human breast cells is uncertain. We studied the expression and function of endogenous LFG in four breast cancer cell lines (MCF-7, MDA-MB-231, T-47D and HS 578T), a human breast epithelial cell line (HS 578Bst), and in healthy and cancerous breast tissues. Molecular (Western blot and RT-PCR) and immunohistochemical techniques were used to investigate the LFG expression. To investigate the breast cancer cell proliferation in the presence of Fas, we performed fluorescent cell viability assays. The possible association of Fas with LFG was analyzed by immunofluorescence microscopy. In this paper, we provide convincing evidence that LFG is overexpressed in several human breast cancer cell lines. More importantly, we found that the LFG expression correlates with high tumour grades in primary breast tumours. Finally, we demonstrated that Fas sensitivity is reduced in breast cancer cell lines expressing LFG. Our results indicated that LFG is strongly expressed in breast cancer epithelial cells. Moreover, the overexpression of LFG correlated with tumour grade and reduced Fas sensitivity. Our findings support the idea that LFG may have a role in the downregulation of apoptosis in breast cancer cells.     

The cellular retinoic-acid-binding protein is expressed in tissues associated with retinoic-acid-induced malformations

Retinoic acid (RA) is thought to play a role in embryonic pattern formation in vertebrates. A naturally occurring gradient of endogenous RA has been demonstrated in the developing chick limb bud, while local application of RA leads to the formation of additional digits. In mammals, a well-defined spectrum of birth defects has been reported as a result of fetal exposure to excess RA. In analogy to the chick limb bud, it may be speculated that these malformations are the result of disturbance of morphogenetic RA concentration gradients. A candidate gene involved in the regulation of endogenous RA concentrations is the gene encoding cellular RA binding protein (CRABP). We have isolated a partial cDNA clone corresponding to the chicken homolog of CRABP, and performed in situ hybridization experiments on sections of embryos at various stages of development. CRABP expression was detected in the CNS, the craniofacial mesenchyme, ganglia of the peripheral nervous system, the limb bud, and the visceral arch area. Our results indicate that the spatiotemporally specified expression pattern displayed by the CRABP gene exhibits a striking correspondence to the tissues that are affected by exposure of avian or mammalian embryos to RA. We hypothesize that CRABP plays an important role in normal embryogenesis and that embryonic tissues showing high CRABP expression are susceptible to the adverse effects of excess RA.     

Human cationic amino acid transporter hCAT-3 is preferentially expressed in peripheral tissues

At least five distinct carrier proteins form the family of mammalian cationic amino acid transporters (CATs). We have cloned a cDNA containing the complete coding region of human CAT-3. hCAT-3 is glycosylated and localized to the plasma membrane. Transport studies in Xenopus laevis oocytes revealed that hCAT-3 is selective for cationic L-amino acids and exhibits a maximal transport activity similar to other CAT proteins. The apparent substrate affinity and sensitivity to trans-stimulation of hCAT-3 resembles most closely hCAT-2B. This is in contrast to rat and murine CAT-3 proteins that have been reported to display a very low activity and to be inhibited by neutral and anionic L-amino acids as well as D-arginine (Hosokawa, H., et al. (1997) J. Biol. Chem. 272, 8717-8722; Ito, K., and Groudine, M. (1997) J. Biol. Chem. 272, 26780-26786). Also, in adult rat and mouse, CAT-3 has been found exclusively in central neurons. Human CAT-3 expression is not restricted to the brain, in fact, by far the highest expression was found in thymus. Also in other peripheral tissues, hCAT-3 expression was equal to or higher than in most brain regions, suggesting that hCAT-3 is not a neuron-specific transporter.