Biosynthesis of intestinal microvillar proteins. Expression of aminopeptidase N along the crypt-villus axis

The expression of pig small-intestinal aminopeptidase N (EC 3.4.11.2) along the crypt-villus axis was studied in tangential sections of [35S]-methionine-labelled, organ-cultured explants. The only detectable molecular forms of aminopeptidase N along the crypt-villus axis were polypeptides of Mr 140 000 and 166 000, representing the enzyme in a transient and mature form respectively. The synthesis was at a very low level in the crypt region in experiments with labelling periods ranging from 10 min to 3 h. These findings indicate that crypt cells are not fully committed to the expression of aminopeptidase N, either in its mature or in any other immunoreactive molecular form. The expression of aminopeptidase N was markedly stimulated by dexamethasone (1 microgram/ml). During labelling periods of 3 h, dexamethasone caused an approximately threefold increase in the expression of the enzyme in the crypt cells and a moderate increase of about 20% in the villus cells. Whereas the latter can possibly be ascribed to a general protective effect of dexamethasone on villus architecture, these experiments indicate that crypt cells of mucosa from adult individuals exhibit the same sensitivity to glucocorticoids as does the intestinal epithelium during the prenatal and early postnatal phase.     

Distribution and biosynthesis of aminopeptidase N and dipeptidyl aminopeptidase IV in rat small intestine

The regional, cellular and subcellular distribution patterns of aminopeptidase N and dipeptidyl aminopeptidase IV were examined in rat small intestine. Aminopeptidase N of brush border membrane had maximal activity in the upper and middle intestine, while dipeptidyl aminopeptidase IV had a more uniform distribution profile with relatively high activity in the ileum. Along the villus and crypt cell gradient, the activity of both enzymes was maximally expressed in the mid-villus cells. However there was substantial dipeptidyl aminopeptidase IV activity in the crypt cells. Both enzymes were primarily associated with brush border membranes in all segments, however, in the proximal intestine, a significant amount of dipeptidyl aminopeptidase IV activity was associated with the cytosol fraction. The cytosol and brush border membrane forms of dipeptidyl aminopeptidase IV were immunologically identical and had the same electrophoretic mobility on disc gels. In contrast, the soluble and brush border membrane-bound forms of aminopeptidase N were immunologically distinct. When the total amount of aminopeptidase N and dipeptidyl aminopeptidase IV was determined by competitive radioimmunoassay, there were no regional or cellular differences in specific activity (enzyme activity/mg of enzyme protein) of either enzyme in brush border membrane and homogenate. The specific activity of both enzymes in a purified Golgi membrane fraction as measured by radioimmunoassay was about half that of the brush border membrane fraction. These results suggest that (1) aminopeptidase N and dipeptidyl aminopeptidase IV have different regional, cellular and subcellular distribution patterns; (2) there are enzymatically inactive forms of both enzymes present in a constant proportion to active molecules and that (3) a two-fold activation of precursor enzyme forms occurs during transfer from the Golgi membranes to the brush border membranes.     

Intestinal enzymes activities in isolated villus and crypt cells during postnatal development of the rat

Summary A modification of Weiser's (1973) cell isolation method was used in order to study the developmental pattern of various intestinal enzyme activities in villus and crypt cells of normal rats from 5 days after birth until 8 weeks. Alkaline phosphatase and enterokinase activities were always located in the upper villus zone during postnatal development. Enterokinase activity was higher in the upper villus cells during the third week of life than after this period. Aminopeptidase activity was located in the crypt cells during the first week, its maximum activity remained in this area until the third week. At this time, sucrase activity appeared in the crypt cells, then aminopeptidase and sucrase activities rose to the villus zone during the fourth week. Amylase activity was detected along the entire crypt-villus axis 5 days after birth, reaching maximum activity in crypt cells at the end of the first week and in the upper villus cells after the fourth week. In contrast with the other enzymes studied almost all amylase activity was soluble in the youngest animals whereas at weaning most of the activity appeared in a particulate form in the villus cells. But in the crypt cells the ratio between particulate and soluble form remained unchanged until the adult stage. Various hypotheses are advanced to explain the patterns of evolution of the different enzymes.     

Amino acid sequence deduced from a rat kidney cDNA suggests it encodes the Zn-peptidase aminopeptidase N

We have isolated and characterized rat kidney cDNA clones encoding a 140-kDa glycoprotein that exhibits characteristics of a cell surface Zn-peptidase. Structural features predicted for this putative kidney Zn-peptidase (KZP) are most consistent with properties previously determined for the Zn-peptidase aminopeptidase N. The deduced amino acid sequence of rat KZP is almost identical to the NH2-terminal sequence of aminopeptidase N purified from rabbit. The overall amino acid composition predicted for rat KZP is remarkably similar to that previously determined for rabbit and pig aminopeptidase N. The predicted Mr of rat kidney KZP approximates the Mr of the unglycosylated form of aminopeptidase N. The topology predicted for KZP is identical to that observed for aminopeptidase N: a short cytoplasmic domain at the NH2 terminus immediately precedes an uncleaved signal/anchor domain; a stalk region connects this membrane anchor to the extracellular, hydrophilic bulk of the protein containing catalytic sites required for Zn-peptidase activity. In addition, mRNA encoding KZP is present in tissues known to exhibit aminopeptidase N activity. Taken together with the observation that only a single gene homologous to KZP DNA is present in the rat and human genomes, these results suggest that we have established the primary structure of rat kidney aminopeptidase N.     

Distribution of Ca2+-ATPase, ATP-dependent Ca2+-transport, calmodulin and vitamin D-dependent Ca2+-binding protein along the villus-crypt axis in rat duodenum

The migration of intestinal epithelial cells from the crypts to the tips of villi is associated with progressive cell differentiation. The changes in Ca2+-ATPase activity and ATP-dependent Ca2+-transport rates in basolateral membranes from rat duodenum were measured during migration along the crypt-villus axis. In addition, vitamin D-dependent calcium-binding protein and calmodulin content were measured in homogenates of six cell populations which were sequentially derived from villus tip to crypt base. Alkaline phosphatase activity was highest at the tip of the villus (fraction I) and decreased more than 20-fold towards the crypt base (fraction VI). (Na+ + K+)-ATPase activity also decreased along the villus-crypt axis but in a less pronounced manner than alkaline phosphatase. ATP-dependent Ca2+-transport in basolateral membranes was highest in fraction II (8.2 +/- 0.3 nmol Ca2+/min per mg protein) and decreased slightly towards the villus tip and base (fraction V). The youngest cells in the crypt had the lowest Ca2+-transport activity (0.9 +/- 0.1 nmol Ca2+/min per mg protein). The distribution of high-affinity Ca2+-ATPase activity in basolateral membranes correlated with the distribution of ATP-dependent Ca2+-transport. The activity of Na+/Ca2+ exchange was equal in villus and crypt basolateral membranes. Compared to the ATP-dependent Ca2+-transport system, the Na+/Ca2+ exchanger is of minor importance in villus cells but may play a more significant role in crypt cells. Calcium-binding protein decreased from mid-villus towards the villus base and was undetectable in crypt cells. Calmodulin levels were equal along the villus-crypt axis. It is concluded that vitamin D-dependent calcium absorption takes primarily place in villus cells of rat duodenum.     

Modulation of xanthine dehydrogenase and oxidase activities during the hormonal induction of vaginal epithelial differentiation in ovariectomized mice

In situ hybridization and immunocytochemical techniques have been used to examine the distribution of vitamin-D-induced calbindin mRNA and calbindin protein in enterocytes lining the crypts and villi of chicken small intestine. Basal villus enterocytes contained approximately twice as much calbindin but over three times as much calbindin mRNA compared to values found in basal crypt and upper villus enterocytes, all values being measured 2 days after vitamin D injection into D-deficient chickens. Virtually no calbindin mRNA was detected in tissues taken from control D-deficient birds. Direct proportionality found between calbindin mRNA and calbindin content in enterocytes of basal crypt, mid and upper villus suggests pre-translational control over calbindin synthesis. The implications of possible inefficient translation of calbindin mRNA in basal villus enterocytes are discussed. Present methods of analysis provide a novel way to study mechanisms controlling gene expression throughout the whole process of enterocyte differentiation.     

Light microscopic localization of hepatic fatty acid binding protein mRNA in jejunal epithelia of rats using in situ hybridization, immunohistochemical, and autoradiographic techniques

An in situ hybridization technique using a [35S]-labeled oligonucleotide probe was employed, in combination with immunohistochemistry and autoradiography, to examine gene expression for hepatic fatty acid binding protein (FABP) in the jejunal epithelia from both fed and fasted rats. In rats fed ad libitum, immunoreactivity and mRNA signal for FABP were localized to the absorptive epithelial cells lining the villus, whereas they were absent in the crypt epithelial cells. The level of FABP mRNA was relatively low in the tip of the villus, although FABP immunoreactivity remained high in this area. Animals fasted for 3 days exhibited a downward shift of the lower boundary of the FABP-expressing cell population into the middle portion of the crypt, in terms of the immunoreactivity and the mRNA signal. The proliferative cell compartment of the crypt, as revealed by [3H]-TdR incorporation, showed no substantial change in size between the fed and fasted states. The present results provided evidence that (a) during the differentiation and upward migration of the absorptive epithelial cells, the expression of FABP gene begins at the crypt-villus junction and declines before the cells reach the villus tip, and (b) fasting induces an earlier expression of the FABP gene in the maturing crypt epithelial cells.     

A strategy for isolation of cDNAs encoding proteins affecting human intestinal epithelial cell growth and differentiation: characterization of a novel gut-specific N-myristoylated annexin

The human intestinal epithelium is rapidly and perpetually renewed as the descendants of multipotent stem cells located in crypts undergo proliferation, differentiation, and eventual exfoliation during a very well organized migration along the crypt to villus axis. The mechanisms that establish and maintain this balance between proliferation and differentiation are largely unknown. We have utilized HT-29 cells, derived from a human colon adenocarcinoma, as a model system for identifying gene products that may regulate these processes. Proliferating HT-29 cells cultured in the absence of glucose (e.g., using inosine as the carbon source) have some of the characteristics of undifferentiated but committed crypt epithelial cells while postconfluent cells cultured in the absence of glucose resemble terminally differentiated enterocytes or goblet cells. A cDNA library, constructed from exponentially growing HT-29 cells maintained in inosine-containing media, was sequentially screened with a series of probes depleted of sequences encoding housekeeping functions and enriched for intestine-specific sequences that are expressed in proliferating committed, but not differentiated, epithelial cells. Of 100,000 recombinant phage surveyed, one was found whose cDNA was derived from an apparently gut-specific mRNA. It encodes a 316 residue, 35,463-D protein that is a new member of the annexin/lipocortin family. Other family members have been implicated in regulation of cellular growth and in signal transduction pathways. RNA blot and in situ hybridization studies indicate that the gene encoding this new annexin exhibits region-specific expression along both axes of the human gut: (a) highest levels of mRNA are present in the jejunum with marked and progressive reductions occurring distally; (b) its mRNA appears in crypt-associated epithelial cells and increases in concentration as they exit the crypt. Villus-associated epithelial cells continue to transcribe this gene during their differentiation/translocation up the villus. Immunocytochemical studies reveal that the intestine-specific annexin (ISA) is associated with the plasma membrane of undifferentiated, proliferating crypt epithelial cells as well as differentiated villus enterocytes. In polarized enterocytes, the highest concentrations of ISA are found at the apical compared to basolateral membrane. In vitro studies using an octapeptide derived from residues 2-9 of the primary translation product of ISA mRNA and purified myristoyl-CoA:protein N-myristoyltransferase suggested that it is N-myristoylated. In vivo labeling studies confirmed that myristate is covalently attached to ISA via a hydroxylamine resistant amide linkage. The restricted cellular expression and acylation of ISA distinguish it from other known annexins.(ABSTRACT TRUNCATED AT 400 WORDS)     

Basolateral membrane lipid dynamics alter Na-K ATPase activity in rabbit small intestine.

The role of basolateral membrane fluidity in regulating Na-K ATPase activity along the crypt-villus axis in rabbit distal small intestine was assessed. Basolateral membranes were prepared from isolated villus and crypt enterocytes at 24- to 28-fold enhancement. Villus basolateral membranes were significantly (p < 0.001) more fluid than crypt basolateral membranes as measured by 1,6-diphenyl-1,3,5-hexatriene. No difference was seen between the two groups as measured by either 2-(9-anthroyloxy)-stearic fatty acid or 16-(9-anthroyloxy)-palmitic acid. Fluidity alterations were accompanied by an increased phospholipid content in villus membranes, which resulted in a decreased cholesterol:phospholipid ratio and an increased lipid:protein molar ratio. Na-K ATPase activity was significantly (p < 0.01) greater in villus basolateral membranes than in crypt membranes, and demonstrated a greater sensitivity to ouabain inhibition. Ouabain inhibition curves calculated from villus data fit well (p < 0.001) with a two binding site model, with a high affinity (Ki 16 nM) and a low affinity (Ki 4.2 microM) ouabain binding site. In crypt basolateral membranes, only a low affinity site was apparent (Ki 3.0 microM). Fluidizing crypt basolateral membranes in vitro with benzyl alcohol to levels seen in villus basolateral membranes resulted in the appearance of a high affinity ouabain binding site (Ki 110 nM) and an increased sensitivity of Na-K ATPase to ouabain inhibition. The fluidization of villus basolateral membranes eliminated the binding associated with the high affinity site. Treatment with methanol, as a control, did not alter Na-K ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Apical Na+-D-glucose cotransporter 1 (SGLT1) activity and protein abundance are expressed along the jejunal crypt-villus axis in the neonatal pig

Gut apical Na(+)-glucose cotransporter 1 (SGLT1) activity is high at the birth and during suckling, thus contributing substantially to neonatal glucose homeostasis. We hypothesize that neonates possess high SGLT1 maximal activity by expressing apical SGLT1 protein along the intestinal crypt-villus axis via unique control mechanisms. Kinetics of SGLT1 activity in apical membrane vesicles, prepared from epithelial cells sequentially isolated along the jejunal crypt-villus axis from neonatal piglets by the distended intestinal sac method, were measured. High levels of maximal SGLT1 uptake activity were shown to exist along the jejunal crypt-villus axis in the piglets. Real-time RT-PCR analyses showed that SGLT1 mRNA abundance was lower (P < 0.05) by 30-35% in crypt cells than in villus cells. There were no significant differences in SGLT1 protein abundances on the jejunal apical membrane among upper villus, middle villus, and crypt cells, consistent with the immunohistochemical staining pattern. Higher abundances (P < 0.05) of total eukaryotic initiation factor 4E (eIF4E) protein and eIE4E-binding protein 1 γ-isoform in contrast to a lower (P < 0.05) abundance of phosphorylated (Pi) eukaryotic elongation factor 2 (eEF2) protein and the eEF2-Pi to total eEF2 abundance ratio suggest higher global protein translational efficiency in the crypt cells than in the upper villus cells. In conclusion, neonates have high intestinal apical SGLT1 uptake activity by abundantly expressing SGLT1 protein in the epithelia and on the apical membrane along the entire crypt-villus axis in association with enhanced protein translational control mechanisms in the crypt cells.     

Circadian rhythm in the rate of cellular proliferation and in the size of the functional compartment of mouse jejunal epithelium

Circadian variation in the small bowel mucosa of male Balb/c mouse was studied. The labelling was studied at 2 hourly intervals throughout a 24 h period by using autoradiographic techniques with 3HTdR. A 12 h light-dark schedule was employed. Villus and crypt cell populations, together with the mitotic index, were studied using the micro-dissection technique. Growth fractions were determined from the labelling index distribution curves. The peaks in both villus and crypt cell population occurred during the day, with maximum villus population of 3,887 cells/villus at 13.00 h and maximum crypt population of 178 cells per crypt at 09.00 h. The peaks of labelling index (Is) and mitotic index (Im) occurred during the dark period. The peak value of Is 38% at 17.00 h-19.00 h, was about 6-8 h in advance of the peak value of Im (6%). The peak in growth fraction corresponded to that of the labelling index.     

Mechanism of inhibition of proton: Dipeptide co-transport during chronic enteritis in the mammalian small intestine

Amino acids, a critical energy source for the intestinal epithelial cells, are more efficiently assimilated in the normal intestine via peptide co-transporters such as proton:dipeptide co-transport (such as PepT1). Active uptake of a non-hydrolyzable dipeptide (glycosarcosine) was used as a substrate and PepT1 was found to be present in normal villus, but not crypt cells. The mRNA for this transporter was also found in villus, but not crypt cells from the normal rabbit intestine. PepT1 was significantly reduced in villus cells also diminished in villus cell brush border membrane vesicles both from the chronically inflamed intestine. Kinetic studies demonstrated that the mechanism of inhibition of PepT1 during chronic enteritis was secondary to a decrease in the affinity of the co-transporter for the dipeptide without an alteration in the maximal rate of uptake (V_max). Northern blot studies also demonstrated unaltered steady state mRNA levels of this transporter in the chronically inflamed intestine. Proton dipeptide transport is found in normal intestinal villus cells and is inhibited during chronic intestinal inflammation. The mechanism of inhibition is secondary to altered affinity of the co-transporter for the dipeptide.     

CVD: the intestinal crypt/villus in situ hybridization database

The intestinal crypt/villus in situ hybridization database (CVD) query interface is a web-based tool to search for genes with similar relative expression patterns along the crypt/villus axis of the mammalian intestine. The CVD is an online database holding information for relative gene expression patterns in the mammalian intestine and is based on the scoring of in situ hybridization experiments reported in the literature. CVD contains expression data for 88 different genes collected from 156 different in situ hybridization profiles. The web-based query interface allows execution of both single gene queries and pattern searches. The query results provide links to the most relevant public gene databases. AVAILABILITY: http://pc113.imbg.ku.dk/ps/