Amino acid transporters expression in acinar cells is changed during acute pancreatitis

Pancreatic acinar cells accumulate amino acids against a marked concentration gradient to synthesize digestive enzymes. Thus, the function of acinar cells depends on amino acid uptake mediated by active transport. Despite the importance of this process, pancreatic amino acid transporter expression and cellular localization is still unclear. We screened mouse pancreas for the expression of genes encoding amino acid transporters. We showed that the most highly expressed transporters, namely sodium dependent SNAT3 (Slc38a3) and SNAT5 (Slc38a5) and sodium independent neutral amino acids transporters LAT1 (Slc7a5) and LAT2 (Slc7a8), are expressed in the basolateral membrane of acinar cells. SNAT3 and SNAT5, LAT1 and LAT2 are expressed in acinar cells. Additional evidence that these transporters are expressed in mature acinar cells was gained using acinar cell culture and acute pancreatitis models. In the acute phase of pancreatic injury, when acinar cell loss occurs, and in an acinar cell culture model, which mimics changes occurring during pancreatitis, SNAT3 and SNAT5 are strongly down-regulated. LAT1 and LAT2 were down-regulated only in the in vitro model. At protein level, SNAT3 and SNAT5 expression was also reduced during pancreatitis. Expression of other amino acid transporters was also modified in both models of pancreatitis. The subset of transporters with differential expression patterns during acute pancreatitis might be involved in the injury/regeneration phases. Further expression, localization and functional studies will follow to better understand changes occurring during acute pancreatitis. These findings provide insight into pancreatic amino acid transport in healthy pancreas and during acute pancreatitis injury.

Defective intestinal amino acid absorption in Ace2 null mice

Mutations in the main intestinal and kidney luminal neutral amino acid transporter B(0)AT1 (Slc6a19) lead to Hartnup disorder, a condition that is characterized by neutral aminoaciduria and in some cases pellagra-like symptoms. These latter symptoms caused by low-niacin are thought to result from defective intestinal absorption of its precursor L-tryptophan. Since Ace2 is necessary for intestinal B(0)AT1 expression, we tested the impact of intestinal B(0)AT1 absence in ace2 null mice. Their weight gain following weaning was decreased, and Na(+)-dependent uptake of B(0)AT1 substrates measured in everted intestinal rings was defective. Additionally, high-affinity Na(+)-dependent transport of L-proline, presumably via SIT1 (Slc6a20), was absent, whereas glucose uptake via SGLT1 (Slc5a1) was not affected. Measurements of small intestine luminal amino acid content following gavage showed that more L-tryptophan than other B(0)AT1 substrates reach the ileum in wild-type mice, which is in line with its known lower apparent affinity. In ace2 null mice, the absorption defect was confirmed by a severalfold increase of L-tryptophan and of other neutral amino acids reaching the ileum lumen. Furthermore, plasma and muscle levels of glycine and L-tryptophan were significantly decreased in ace2 null mice, with other neutral amino acids displaying a similar trend. A low-protein/low-niacin diet challenge led to differential changes in plasma amino acid levels in both wild-type and ace2 null mice, but only in ace2 null mice to a stop in weight gain. Despite the combination of low-niacin with a low-protein diet, plasma niacin concentrations remained normal in ace2 null mice and no pellagra symptoms, such as photosensitive skin rash or ataxia, were observed. In summary, mice lacking Ace2-dependent intestinal amino acid transport display no total niacin deficiency nor clear pellagra symptoms, even under a low-protein and low-niacin diet, despite gross amino acid homeostasis alterations.

The tegument of the human parasitic worm Schistosoma mansoni as an excretory organ: the surface aquaporin SmAQP is a lactate transporter

Adult schistosomes are intravascular parasites that metabolize imported glucose largely via glycolysis. How the parasites get rid of the large amounts of lactic acid this generates is unknown at the molecular level. Here, we report that worms whose aquaporin gene (SmAQP) has been suppressed using RNAi fail to rapidly acidify their culture medium and excrete less lactate compared to controls. Functional expression of SmAQP in Xenopus oocytes demonstrates that this protein can transport lactate following Michaelis-Menten kinetics with low apparent affinity (Km = 41±5. 8 mM) and with a low energy of activation (E_a = 7.18±0.7 kcal/mol). Phloretin, a known inhibitor of lactate release from schistosomes, also inhibits lactate movement in SmAQP-expressing oocytes. In keeping with the substrate promiscuity of other aquaporins, SmAQP is shown here to be also capable of transporting water, mannitol, fructose and alanine but not glucose. Using immunofluorescent and immuno-EM, we confirm that SmAQP is localized in the tegument of adult worms. These findings extend the proposed functions of the schistosome tegument beyond its known capacity as an organ of nutrient uptake to include a role in metabolic waste excretion.

Orphan transporter SLC6A18 is renal neutral amino acid transporter B0AT3

The orphan transporter Slc6a18 (XT2) is highly expressed at the luminal membrane of kidney proximal tubules and displays approximately 50% identity with Slc6a19 (B(0)AT1), which is the main neutral amino acid transporter in both kidney and small intestine. As yet, the amino acid transport function of XT2 has only been experimentally supported by the urinary glycine loss observed in xt2 null mice. We report here that in Xenopus laevis oocytes, co-expressed ACE2 (angiotensin-converting enzyme 2) associates with XT2 and reveals its function as a Na(+)- and Cl(-)-de pend ent neutral amino acid transporter. In contrast to its association with ACE2 observed in Xenopus laevis oocytes, our experiments with ace2 and collectrin null mice demonstrate that in vivo it is Collectrin, a smaller homologue of ACE2, that is required for functional expression of XT2 in kidney. To assess the function of XT2 in vivo, we reanalyzed its knock-out mouse model after more than 10 generations of backcrossing into C57BL/6 background. In addition to the previously published glycinuria, we observed a urinary loss of several other amino acids, in particular beta-branched and small neutral ones. Using telemetry, we confirmed the previously described link of XT2 absence with hypertension but only in physically restrained animals. Taken together, our data indicate that the formerly orphan transporter XT2 functions as a sodium and chloride-de pend ent neutral amino acid transporter that we propose to rename B(0)AT3.

Tissue-specific amino acid transporter partners ACE2 and collectrin differentially interact with hartnup mutations

BACKGROUND & AIMS

Hartnup amino acid transporter B(0)AT1 (SLC6A19) is the major luminal sodium-dependent neutral amino acid transporter of small intestine and kidney proximal tubule. The expression of B(0)AT1 in kidney was recently shown to depend on its association with collectrin (Tmem27), a protein homologous to the membrane-anchoring domain of angiotensin-converting enzyme (ACE) 2.

METHODS

Because collectrin is almost absent from small intestine, we tested the hypothesis that it is ACE2 that interacts with B(0)AT1 in enterocytes. Furthermore, because B(0)AT1 expression depends on an associated protein, we tested the hypothesis that Hartnup-causing B(0)AT1 mutations differentially impact on B(0)AT1 interaction with intestinal and kidney accessory proteins.

RESULTS

Immunofluorescence, coimmunoprecipitation, and functional experiments using wild-type and ace2-null mice showed that expression of B(0)AT1 in small intestine critically depends on ACE2. Coexpressing new and previously identified Hartnup disorder-causing missense mutations of B(0)AT1 with either collectrin or ACE2 in Xenopus laevis oocytes showed that the high-frequency D173N and the newly identified P265L mutant B(0)AT1 transporters can still be activated by ACE2 but not collectrin coexpression. In contrast, the human A69T and R240Q B(0)AT1 mutants cannot be activated by either of the associated proteins, although they function as wild-type B(0)AT1 when expressed alone.

CONCLUSIONS

We thus show that ACE2 is necessary for the expression of the Hartnup transporter in intestine and suggest that the differential functional association of mutant B(0)AT1 transporters with ACE2 and collectrin in intestine and kidney, respectively, participates in the phenotypic heterogeneity of human Hartnup disorder.

Amino acid transport in schistosomes: Characterization of the permeaseheavy chain SPRM1hc

Schistosomes are human parasitic flatworms that constitute an important public health problem globally. Adult parasites live in the bloodstream where they import nutrients such as amino acids across their body surface (the tegument). One amino acid transporter, Schistosome Permease 1 light chain, SPRM1lc, a member of the glycoprotein-associated family of transporters (gpaAT), has been characterized in schistosomes. Only a single member of the SLC3 family of glycoproteins that associate with gpaATs is found following extensive searching of the genomes of Schistosoma mansoni and S. japonicum. In this report, we characterize this schistosome permease heavy chain (SPRM1hc) gene and protein. The 72-kDa gene product is predicted to possess a single transmembrane domain, a (betaalpha)(8) (TIM barrel) conformation and a catalytic triad. Xenopus oocytes functionally expressing SPRM1hc with SPRM1lc import phenylalanine, arginine, lysine, alanine, glutamine, histidine, tryptophan, and leucine. Biochemical characterization demonstrates that in Xenopus extracts and in schistosome extracts SPRM1hc is associated into a high molecular weight complex with SPRM1lc that is disrupted by reducing agents. Quantitative real-time PCR and Western analysis demonstrate that SPRM1hc is expressed in each schistosome life stage examined (eggs, cercariae, schistosomula, adult males and females). SPRM1hc is widely distributed throughout adult male and female worms as determined by immunolocalization. Consistent with the hypothesis that SPRM1hc functions to facilitate nutrient uptake from host blood, immunogold electron microscopy confirms that the protein is distributed on the host-interactive tegumental membranes. We propose that surface-exposed, host-interactive, nutrient-transporting proteins like the SPRM1 heterodimer are promising vaccine candidates.

Preferred transport of O-(2-[18F]fluoroethyl)-d-tyrosine (d-FET) into the porcine brain

Amino acids are valuable tracers for brain tumor imaging with positron emission tomography (PET). In this study the transport of O-(2-[(18)F]fluoroethyl)-D-tyrosine (D-FET) across the blood-brain barrier (BBB) was studied with PET in anesthetized piglets and patients after subtotal resection of brain tumors and compared with O-(2-[(18)F]fluoroethyl)-L-tyrosine (L-FET) and 3-O-methyl-6-[(18)F]fluoro-L-DOPA (L-OMFD). In piglets, compartmental modeling of PET data was used to calculate the rate constants for the blood-brain (K(1)) and the brain-blood (k(2)) transfer of D-FET, L-FET and L-OMFD. In patients standardized uptake values (SUVs) were calculated in brain cortex and lesions. Additionally, affinity determinations on various amino acid transporters (LAT1, LAT2, PAT1, XPCT) were performed in vitro using unlabeled D-FET, L-FET and L-OMFD. The initial brain uptake of D-FET in piglets was more than two-fold higher than that of l-FET, whereas the initial brain uptake of D-FET in patients was similar to that of L-FET. Calculation of K(1) and k(2) from the brain uptake curves and the plasma input data in piglets revealed about 4- and 2-fold higher values for D-FET compared to L-FET and L-OMFD, respectively. The distribution volume of D-FET in the piglet brain was slightly higher than that of L-FET as it was also found for most other organs. In brain tumor patients, initial D-FET uptake in the brain was similar to that of L-FET but showed faster tracer washout. L-FET uptake remained rather constant and provided a better delineation of residual tumor than D-FET. In conclusion, our data indicate considerable differences of stereoselective amino acid transport at the BBB in different species. Therefore, the results from animal experiments concerning BBB amino acid transport may not be transferable to humans.

Reactivity of human natural antibodies to endothelial cells from Galalpha(1,3)Gal-deficient pigs

BACKGROUND

Xenoreactive human natural antibodies (NAb) are predominantly directed against galactose-alpha(1,3)galactose (Gal). Binding of immunoglobulin (Ig) G and IgM NAb activates porcine endothelial cells (pEC) and triggers complement lysis responsible for hyperacute xenograft rejection. In vitro, IgG NAb induce human natural killer (NK) cell-mediated lysis of pEC by antibody-dependent cell-mediated cytotoxicity (ADCC). The present study examined the levels of anti-porcine NAb in a large number of individuals and addressed the functional role of non-Gal anti-porcine NAb.

METHODS

Sera from 120 healthy human blood donors were analyzed for the presence of anti-porcine NAb by flow cytometry using porcine red blood cells (pRBC), lymphoblastoid cells (pLCL), and pEC derived from control or Gal-deficient pigs. Xenogeneic complement lysis was measured by flow cytometry using human serum and rabbit complement. ADCC was analyzed by chromium-release assays using human serum and freshly isolated NK cells.

RESULTS

Human IgM binding to pRBC was found in 93% and IgG binding in 86% of all samples. Non-Gal NAb comprised 13% of total IgM and 36% of total IgG binding to pEC. NAb/complement-induced lysis and ADCC of Gal-deficient compared to Gal-positive pEC were 21% and 29%, respectively. The majority of anti-Gal and non-Gal IgG NAb were of the IgG2 subclass.

CONCLUSIONS

The generation of Gal-deficient pigs has overcome hyperacute anti-Gal-mediated xenograft rejection in nonhuman primates. Non-Gal anti-porcine NAb represent a potentially relevant immunological hurdle in a subgroup of individuals by inducing endothelial damage in xenografts.

Isotype-specific detection of ABO blood group antibodies using a novel flow cytometric method

Several methods to detect anti-A/B antibodies based on haemagglutination and haemolysis have been described. These methods measure predominantly anti-A/B immunoglobulin (Ig)M, whereas anti-A/B IgG and IgG subclasses are less well examined. We established a flow cytometry method (ABO-fluorescence-activated cell sorting; ABO-FACS) to quantify binding of anti-A/B IgM, IgG and IgG subclasses to human A or B red blood cells. Anti-A/B IgM were present in the majority of 120 blood donors, as expected from blood group typing. The sensitivity and specificity of anti-A/B IgM to predict the blood group was 93% and 96% respectively. Anti-A/B IgG was found in 34/38 blood group O samples (89%). Anti-B IgG in blood group A or anti-A IgG in blood group B was present in 4/28 (14%) and 1/28 (4%) samples, respectively, and absent in 26 AB sera. IgG2 was the predominant IgG subclass. The correlation of anti-A/B IgM and IgG in the ABO-FACS with haemagglutination titres was 0.870 and 0.783, respectively (n = 240; P < 0.001) whereas the comparison of ABO-FACS with ABO-enzyme-linked immunosorbent assay was less significant. In conclusion, ABO-FACS is a valid method to quantify anti-A/B IgM, IgG and IgG subclasses. It opens the possibility of isotype-specific monitoring of anti-A/B antibodies levels after ABO-incompatible solid organ and stem cell transplantation.

Levels of Anti-A/B Antibodies After ABO-Incompatible Hematopoietic Stem Cell Transplantation

In contrast to solid organ transplantation, ABO incompatibility is generally not associated with survival differences in hematopoietic stem cell transplantation. Therefore, patients receiving ABO-incompatible stem cell transplantation can be analyzed to study the mechanism of tolerance induction after antigen-mismatched transplantation. The goal of this study was to analyze the levels of anti-A/B antibodies after ABO-incompatible transplantation. Host-derived antidonor antibodies disappeared rapidly after transplantation and did not reappear in the further posttransplant course. Donor-derived antihost antibodies did not significantly increase and compatible anti-A/B antibodies remained positive after hematopoietic stem cell transplantation. Thus, there is no evidence for stimulation of donor B lymphocytes to produce antirecipient antibodies suggesting a potential B cell tolerance.

The angiogenesis inhibitor endostatin impairs blood vessel maturation during wound healing

Endostatin is a cleavage product of collagen XVIII that strongly inhibits tumor angiogenesis. To determine if endostatin affects other angiogenic processes, we generated full-thickness excisional wounds on the back of mice that were systemically treated with recombinant murine endostatin. No macroscopic abnormalities of the wound healing process were observed. Histological analysis revealed normal wound contraction and re-epithelialization, but a slight reduction in granulation tissue formation and reduced matrix deposition at the wound edge. The blood vessel density in the wounds of endostatin-treated mice was not affected. However, ultrastructural analysis demonstrated severe abnormalities in blood vessel maturation. The wound vessels in the endostatin-treated mice were narrowed or closed with an irregular luminal surface, resulting in a severe reduction in the number of functional vessels and extravasation of erythrocytes. Endostatin treatment did not affect the expression level and localization of collagen XVIII mRNA and protein. Furthermore, the angiogenesis regulators vascular endothelial growth factor, angiopoietin-1, and angiopoietin-2 were normally expressed in the wounds of endostatin-treated mice. However, expression of the major wound matrix proteins fibronectin and collagens I and III was significantly reduced. This reduction is likely to explain the reduced density of the wound matrix. Our results demonstrate that endostatin treatment reduces the number of functional blood vessels and the matrix density in the granulation tissue, but does not significantly affect the overall wound healing process.

Expression cloning of GABA(B) receptors uncovers similarity to metabotropic glutamate receptors

GABA (gamma-amino-butyric acid), the principal inhibitory neurotransmitter in the brain, signals through ionotropic (GABA(A)/ GABA(c)) and metabotropic (GABA(B)) receptor systems. Here we report the cloning of GABA(B) receptors. Photoaffinity labelling experiments suggest that the cloned receptors correspond to two highly conserved GABA(B) receptor forms present in the vertebrate nervous system. The cloned receptors negatively couple to adenylyl cyclase and show sequence similarity to the metabotropic receptors for the excitatory neurotransmitter L-glutamate.

The two subunits of the asialoglycoprotein receptor contain different sorting information

The human asialoglycoprotein receptor, an endocytic transport receptor of the basolateral surface of hepatocytes, is a hetero-oligomer of two homologous subunits H1 and H2. The cytoplasmic domain of H1 has been shown previously to contain a tyrosine-based signal for endocytosis and basolateral sorting. Here, we have investigated sorting determinants within subunit H2 and their contribution to the targeting of the hetero-oligomeric receptor complex. Despite extensive sequence homology, H2 expressed separately in fibroblast cells was endocytosed poorly, and mutation of phenylalanine 5 (corresponding to the critical tyrosine in H1) did not further reduce internalization. Consistent with this observation, ligand uptake by receptors composed of H1 lacking tyrosine 5 and H2 was inefficient. With respect to polarized transport in Madin-Darby canine kidney cells, H2 could not be analyzed separately, because in the absence of H1 subunit H2 was completely degraded intracellularly. Coexpression of both subunits yielded ligand-binding receptors located specifically on the basolateral surface. The mutant H1(5A) (tyrosine 5 replaced by alanine) is approximately 55% apical in the absence of H2. In cells expressing H1(5A) together with H2, however, subunit H2 directed receptor complexes exclusively to the basolateral domain. Phenylalanine 5 is not essential for basolateral transport. Thus, whereas the endocytosis signal of the hetero-oligomeric asialoglycoprotein receptor resides exclusively in subunit H1, polarized transport to the basolateral domain of Madin-Darby canine kidney cells may involve two signals, only one of which is active for endocytosis.

Related signals for endocytosis and basolateral sorting of the asialoglycoprotein receptor

The major subunit of the human asialoglycoprotein receptor contains signals for efficient endocytosis and specific basolateral expression in polarized Madin-Darby canine kidney cells, both of which are located within its 40-residue cytoplasmic domain. The aromatic residue in this segment, tyrosine 5, which is necessary for efficient clustering into clathrin-coated pits at the plasma membrane, is also necessary for exclusive basolateral delivery. Mutation of this residue to alanine resulted in a nonpolar expression of the protein. Replacement of tyrosine 5 with phenylalanine yielded almost wild-type rates of endocytosis as well as specific basolateral expression, indicating that tyrosine phosphorylation is not essential for either sorting step. The close similarity between the two sorting signals was further corroborated by deletion mutants showing that the amino-terminal 10 residues of the cytoplasmic domain are sufficient for basolateral polarity and efficient endocytosis. The kinetics of appearance of newly synthesized wild-type and mutant receptor protein at the apical and basolateral surfaces indicate that these proteins are sorted intracellularly and are transported directly to the respective domains. Mutants affected in basolateral sorting lost polarity, i.e, appeared to similar extents on both surfaces, indicating that there is no significant apical sorting information elsewhere in the protein. The close correlation between endocytosis and basolateral polarity suggests common recognition mechanisms at the plasma membrane and in the trans-Golgi network.

Growth-promoting effect of a protein-free hemodialysate used in situations of hypoxia and for tissue repair as measured via stimulation of S6-kinase

Solcoseryl is the low-molecular weight fraction of calf blood as manufactured by counterflow dialysis. This hemodialysate (HD) is in clinical use in situations involving hypoxia and for the normalization of tissue repair. The influence of the HD on ZR-75 cells was tested. These cells preferably express receptors for Epidermal Growth Factor (EGF)/Transforming Growth Factor alpha (TGF-alpha) or Somatomedin C (SMC = insulin like growth factor I resp. ILA-I) and react upon stimulation by enhancement of their S6-kinase activity, the latter being a prerequisite for growth. The functional presence of one or several of these peptide growth factors should therefore reflect in a stimulation of S6-kinase activity.