Enteric Nervous System: Identification of a Novel Neuronal Sensory Network in the Duodenal Epithelium.

The communication between the intestinal epithelium and the enteric nervous system has been considered indirect. Mechanical or chemical stimuli activate enteroendocrine cells inducing hormone secretion, which act on sub-epithelial nerve ends, activating the enteric nervous system. However, we identified an epithelial cell that expresses NKAIN4, a neuronal protein associated with the ß-subunit of Na+/K+-ATPase. This cell overexpresses Na+/K+-ATPase and ouabain-insensitive Na+-ATPase, enzymes involved in active sodium transport. NKAIN4-positive cells also express neuronal markers as NeuN, acetylcholine-esterase, acetylcholine-transferase, α3- and α7-subunits of ACh receptors, glutamic-decarboxylase, and serotonin-receptor-7, suggesting they are neurons. NKAIN4-positive cells show a polarized shape with an oval body, an apical process finished in a knob-like terminal in contact with the lumen, a basal cilia body at the base of the apical extension, and basal axon-like soma projections connecting sub-epithelial nerve terminals, lymphoid nodules, glial cells, and enterochromaffin cells, forming a network that reaches the epithelial surface. We also showed, using retrograde labeling and immunofluorescence, that these cells receive afferent signals from the enteric nervous system. Finally, we demonstrated that acetylcholine activates NKAIN4-positive cells inducing Ca2+ mobilization and probably serotonin secretion in enterochromaffin cells. NKAIN4-positive cells are neurons that would form a part of a duodenal sensory network for physiological or noxious luminal stimuli.

The second sodium pump: from the function to the gene.

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Isolation and cloning of the K+-independent, ouabain-insensitive Na+-ATPase.

Primary Na+ transport has been essentially attributed to Na+/K+ pump. However, there are functional and biochemical evidences that suggest the existence of a K+-independent, ouabain-insensitive Na+ pump, associated to a Na+-ATPase with similar characteristics, located at basolateral plasma membrane of epithelial cells. Herein, membrane protein complex associated with this Na+-ATPase was identified. Basolateral membranes from guinea-pig enterocytes were solubilized with polyoxyethylene-9-lauryl ether and Na+-ATPase was purified by concanavalin A affinity and ion exchange chromatographies. Purified enzyme preserves its native biochemical characteristics: Mg2+ dependence, specific Na+ stimulation, K+ independence, ouabain insensitivity and inhibition by furosemide (IC50: 0.5 mM) and vanadate (IC50: 9.1 µM). IgY antibodies against purified Na+-ATPase did not recognize Na+/K+-ATPase and vice versa. Analysis of purified Na+-ATPase by SDS-PAGE and 2D-electrophoresis showed that is constituted by two subunits: 90 (α) and 50 (ß) kDa. Tandem mass spectrometry of α-subunit identified three peptides, also present in most Na+/K+-ATPase isoforms, which were used to design primers for cloning both ATPases by PCR from guinea-pig intestinal epithelial cells. A cDNA fragment of 1148 bp (atna) was cloned, in addition to Na+/K+-ATPase α1-isoform cDNA (1283 bp). In MDCK cells, which constitutively express Na+-ATPase, silencing of atna mRNA specifically suppressed Na+-ATPase α-subunit and ouabain-insensitive Na+-ATPase activity, demonstrating that atna transcript is linked to this enzyme. Guinea-pig atna mRNA sequence (2787 bp) was completed using RLM-RACE. It encodes a protein of 811 amino acids (88.9 kDa) with the nine structural motifs of P-type ATPases. It has 64% identity and 72% homology with guinea-pig Na+/K+-ATPase α1-isoform. These structural and biochemical evidences identify the K+-independent, ouabain-insensitive Na+-ATPase as a unique P-type ATPase.

Purification and characterization of the ouabain-sensitive H+/K+-ATPase from guinea-pig distal colon.

Distal colon absorbs K+ through a Na+-independent, ouabain-sensitive H+/K+-exchange, associated to an apical ouabain-sensitive H+/K+-ATPase. Expression of HKalpha2, gene associated with this ATPase, induces K+-transport mechanisms, whose ouabain susceptibility is inconsistent. Both ouabain-sensitive and ouabain-insensitive K+-ATPase activities have been described in colonocytes. However, native H+/K+-ATPases have not been identified as unique biochemical entities. Herein, a procedure to purify ouabain-sensitive H+/K+-ATPase from guinea-pig distal colon is described. H+/K+-ATPase is Mg2+-dependent and activated by K+, Cs+ and NH4+ but not by Na+ or Li+, independently of K+-accompanying anion. H+/K+-ATPase was inhibited by ouabain and vanadate but insensitive to SCH-28080 and bafilomycin-A. Enzyme was phosphorylated from [32P]-gamma-ATP, forming an acyl-phosphate bond, in an Mg2+-dependent, vanadate-sensitive process. K+ inhibited phosphorylation, effect blocked by ouabain. H+/K+-ATPase is an alpha/beta-heterodimer, whose subunits, identified by Tandem-mass spectrometry, seems to correspond to HKalpha2 and Na+/K+-ATPase beta1-subunit, respectively. Thus, colonic ouabain-sensitive H+/K+-ATPase is a distinctive P-type ATPase.

Modulation of membrane type 1 matrix metalloproteinase by LPS and gamma interferon bound to extracellular matrix in intestinal crypt cells.

Membrane type 1 matrix metalloproteinase (MT1-MMP) is an integral membrane protein that participates in the processing and degradation of cell surface proteins and the extracellular matrix (ECM). This enzyme regulates ECM turnover in wound repair, promotes cell migration and activates other MMPs, such as MMP-2, which is involved in angiogenesis, cell migration and tumoral metastasis. An increase in pro-inflammatory cytokine expression, such as gamma interferon (IFN-gamma), has been associated with chronic wounds in inflammatory bowel diseases. However, the extent to which cytokines modulate MT1-MMP has not been totally defined. In this report, the effects of the bacterial lipopolysaccharide (LPS) and ECM-bound IFN-gamma on MT1-MMP expression and MMP-2 activity were evaluated by Western blot, RT-PCR and zymography in isolated intestinal epithelial and cultured HT-29 cells. In the presence of LPS, ECM-bound IFN-gamma, but not soluble IFN-gamma, reduced the enterocyte MT1-MMP protein expression. In addition, the active form of MMP-2 was also decreased in the presence of both LPS and IFN-gamma, indicating that lower MMP-2 activity accompanied the decrease in MT1-MMP expression. These results suggest the possibility that endotoxin and ECM-bound IFN-gamma may affect matrix remodeling by modulating matrix metalloproteinase in enterocytes during wound healing.

La segunda bomba de sodio: de la proteína al gen / The second sodium pump: from protein to gene

En la membrana laterobasal del epitelio del intestino delgado y del túbulo renal proximal han sido descrito dos mecanismos diferentes de transporte activo primario de Na+: (1) uno dependiente de K+, sensible a la ouabaina e insensible a la furosemida, correspondiente a la bomba de Na+/K+; y (2) otro independiente de K+, insensible a la ouabaina pero inhibida por furosemida, el cual es referido como la segunda bomba de sodio. De igual modo, dos actividades ATPásicas, dependientes de Mg2+, estimuladas por Na+ e inhibidas por vanadato, han sido descritas en esta membrana: (1) una dependiente de K+, sensible a la ouabaina e insensible a la furosemida, correspondiente a la ATPasa de Na+/K+; y (2) otra independiente de K+, insensible a la iuabaina pero inhibida por furosemida, la cual ha sido denominada como la ATPasa de Na+. Dadas las similitudes bioquímicas, se considera que la bomba de Na+/K+ y la segunda bomba de sodio están asociadas con las ATPasas de Na+/K+ y de Na+, respectivamente, como una entidad bioquímica única. No obstante, no había sido posible la separación óptima de ambas enzimas. Recientemente, se logró solubilizar ambas ATPasas utilizando un detergente no-iónico (C12E9), preservando sus actividades, y purificar la ATPasa de Na+ por selección negativa a través de una cromatografía de afinidad con Concanavalina-A-sefarosa. La ATPasa de Na+ esta constituida por dos subunidades: una subunidad alfa de 98 KDa y una subunidad beta de 50 KDa. La subunidad alfa fue parcialmente secuenciada por espectrometría de masa en serie, identificándose tres péptidos que están presentes en la subunidad alfa1 de la ATPasa de Na+/K+. La formación de intermediarios fosforilados durante el ciclo de reacción de la ATPasa de Na+, así como su dependencia de Mg2+ y sensibilidad a vanadato, identifican a esta enzima como integrante de las ATPasas tipo P.

Basolateral plasma membranes of small intestine and proximal renal tubule present two active Na+-transportmechanisms: (1) The Na+/K+-pump, which depends on K+, is inhibited by ouabain but insensitive to furosemide and (2) The Second sodium pump, which is K+-independent, insensitive to ouabain but inhibited by furosemide. Thse two transport mechanisms have been associated with two different Mg2+-dependent Na+-ATPases, inhibited by vanadate: (1) The K+-dependent Na+/K+-ATPase, sensitive to ouabain and insensitive to furosemide, and (2) The K+ independent, Na+-ATPase, which is inhibitable by furosemide and insensitiveto ouabain. There exist multiple biochemical and functional evidences indicating that these two ATPases are different but only recently it has been possible to identify the Na+-ATPase as a unique biochemical entity. Both ATPases can be solubilized in an active form using C12E9 as detergent and separated by exclusion chromatography in sepharose 6-B and affinity chromatography in concanavalinA-sepharose. The Na+-ATPase is constituted by two sub-units: an alpha subunit of 98 KDa and a beta subunit of 50 KDa. The alpha subunit was partially sequenced by Tandem Mass Spectrometry, evidenced three peptides that are also present in the alpha1 subunit of the Na+/K+-ATPase. Na+-ATPase is Mg2+-dependent, inhibited by vanadate and forms phosphorylated intermediate during its reaction cycle ATP, indicating that it si a P-type ATPase. These facts induced us to design degenerated primers against the most preserves motifs present in these ATPases and to intent the cloning of the Na+-ATPase. Thus, we identified a cDNA for a new P-type ATPase probably related with this enzyme.

Backdoor phosphorylation of basolateral plasma membranes of small intestinal epithelial cells: characterization of a furosemide-induced phosphoprotein related to the second sodium pump.

Enterocyte has two different Na+-stimulated ATPases, the ouabain-sensitive Na+/K+ ATPase and a furosemide-inhibitable Na+ ATPase. To identify the polypeptide associated with the Na+-ATPase, 32Pi phosphorylation into basolateral membranes of enterocyte was investigated. Both, ouabain and furosemide induced Mg2+-dependent, vanadate-sensitive 32Pi incorporation into a 100kDa polypeptide. K(m) for Pi was 17.7+/-1.82 microM and 16.8+/-0.69 microM for ouabain-induced and furosemide-induced phosphorylation, respectively. K(m) for furosemide was 1.3+/-0.21 mM. Furosemide-induced 32Pi incorporation was sensitive to alkaline pH and hydroxylamine suggesting an acyl-phosphate bond. Na+ and K+ inhibited 32Pi incorporation induced by ouabain. In contrast, Na+ stimulated furosemide-induced phosphorylation with a K(m) of 16.5+/-5.59 mM while K+ had no effect. Purified Na+/K+ ATPase only presented ouabain-induced phosphoprotein, indicating that furosemide-induced phosphorylation is not related to this enzyme and appears to correspond to a new member of P-type ATPases associated with the second Na+ pump.

Glutamine transport in isolated epithelial intestinal cells. Identification of a Na+-dependent transport mechanism, highly specific for glutamine.

L-glutamine transport was evaluated in isolated cells from the guinea-pig small intestine by measuring [(3)H]- L-glutamine uptake. Villous and crypt cells expressed Na(+)-dependent and Na(+)-independent transport mechanisms. Glutamine transport systems were identified using various amino acids and analogues as inhibitors. In both villous and crypt cells, 2-(methylamino)-isobutyrate (MeAIB), a system A inhibitor, did not inhibit Na(+)-dependent glutamine influx. 2-Aminobicyclo(2,2,1)heptane-2-carboxylate (BCH), a system B(0) and B(0,+) substrate, had no effect on Na(+)-dependent influx. Serine, cysteine and threonine, system ASC inhibitors, reduced Na(+)-dependent influx by 50%. Asparagine, but not histidine, system N inhibitors, reduced Na(+)-dependent glutamine influx by 50%, however the effect of asparagine was not additive to that of threonine. The remaining Na(+)-dependent glutamine influx (50%) was only inhibited by glutamine itself, by Na(+) substitution ( N-methyl-glucamine, K(+), Li(+)) or by external pH reduction. Phenyl-acetyl-glutamine (PAG), a synthetic amino acid analogue, also inhibited this Na(+)-dependent, threonine-insensitive glutamine influx (IC(50) 2.45 mM). The Na(+)-independent uptake was partially inhibited by BCH, a system L inhibitor, and other neutral amino acids, but was not affect by PAG. Our results suggest that glutamine is transported in both villous and crypt cells by the Na(+)-independent system L, by the Na(+)-dependent system ASC and by an as yet undescribed Na(+)-dependent transport mechanism, highly specific for glutamine.