Processing of human pro-lactase-phlorizin hydrolase at reduced temperatures: cleavage is preceded by complex glycosylation

Structural determinants for transport of lactase phlorizin-hydrolase in the early secretory pathway as a multi-domain membrane glycoprotein

BACKGROUND

Lactase phlorizin-hydrolase (LPH) is a membrane anchored type I glycoprotein of the intestinal epithelium that is composed of four homologous structural domains. The role of each distinct domain in the intramolecular organization and function of LPH is not completely understood.

METHODS

Here, we analyzed the early events of LPH biosynthesis and trafficking by directed restructuring of the domain compositions.

RESULTS

Removal of domain I (LPH∆1) results in a malfolded ER-localized protein. By contrast, LPH without domain II (LPH∆2) is normally transported along the secretory pathway, but does not dimerize nor is enzymatically active. Interestingly a polypeptide stretch in domain II between L⁷³⁵-R⁸⁶⁸ exerts an intriguing role in modulating the trafficking behavior of LPH and its biological function. In fact, association of this stretch with transport-competent LPH chimeras results in their ER-arrest or aberrant trafficking. This stretch harbors a unique N-glycosylation site that is responsible for LPH retention in the ER via association with calnexin and facilitates proper folding of domains I and III before ER exit of LPH. Notably, a similar N-glycosylation site is also found in domain IV with comparable effects on the trafficking of LPH-derived molecules.

CONCLUSIONS

Our study provides novel insights into the intramolecular interactions and the sequence of events involved in the folding, dimerization and transport of LPH.

GENERAL SIGNIFICANCE

Elucidation of the structural-functional relevance of the domains in pro-LPH is crucial in unravelling and understanding the molecular basis of carbohydrate malabsorption disorders that are associated with lactase deficiency or lactase malfunction.

Adult-type hypolactasia and regulation of lactase expression

A common genetically determined polymorphism in the human population leads to two distinct phenotypes in adults, lactase persistence and adult-type hypolactasia (lactase non-persistence). All healthy newborn children express high levels of lactase and are able to digest large quantities of lactose, the main carbohydrate in milk. Individuals with adult-type hypolactasia lose their lactase expression before adulthood and consequently often become lactose intolerant with associated digestive problems (e.g. diarrhoea). In contrast, lactase persistent individuals have a lifelong lactase expression and are able to digest lactose as adults. Lactase persistence can be regarded as the mutant phenotype since other mammals down-regulate their lactase expression after weaning (the postweaning decline). This phenomenon does not occur in lactase persistent individuals. The regulation of lactase expression is mainly transcriptional and it is well established that adult-type hypolactasia is inherited in an autosomal recessive manner, whereas persistence is dominant. The recent findings of single nucleotide polymorphisms associated with lactase persistence have made it possible to study the potential mechanisms underlying adult-type hypolactasia. This work has led to the identification of gene-regulatory sequences located far from the lactase gene (LCT). The present review describes the recent advances in the understanding of the regulation of lactase expression and the possible mechanisms behind adult-type hypolactasia.

Regulation of the intestinal glycoprotein glycosylation during postnatal development: role of hormonal and nutritional factors

This review focuses on the regulation of the glycoprotein glycosylation process in small intestine and colon during postnatal development. Glycoproteins play a prominent part in intestine as mucins secreted by the goblet cells and as molecules of biological interest largely present in the microvillus membrane of the enterocytes (digestive enzymes, transporters). The age-related changes in the intestinal glycosylation control the quality of glycan chains of glycoproteins. Postnatal maturation is observed at all stages of the glycoprotein glycosylation. But it is essentially characterised in the external glycosylation by a shift from sialylation to fucosylation depending on the transcriptional regulation of the corresponding glycosyltransferases, but also on coordinate changes in the activities of glycosyltransferases and of their regulatory proteins, in nucleotide-sugar bioavailability and in product degradation by oxidases. Many factors have been evoked to trigger these changes, among which are hormonal (glucocorticoids, insulin) and dietary factors. Changes in the structure of the glycoprotein glycans might be important for the transport, the barrier function, the implantation of the immune defences and of the microflora and even probably for the biological activity of some digestive enzymes.

Processing of human intestinal prolactase to an intermediate form by furin or by a furin-like proprotein convertase

Human lactase-phlorizin hydrolase (human-LPH) is synthesized as a large precursor (prepro-LPH), then cleaved to a pro-LPH of 220 kDa which is further cut to a "mature-like LPH" of a size close to that of mature LPH, i.e. about 150 kDa (in the processing of rabbit pro-LPH the intermediate has a mass of approximately 180 kDa). By coexpression of human prepro-LPH with furin in COS-7 cells we show that furin generates a mature-like LPH. Radioactive amino acid sequence analysis reveals that furin recognizes the motif R-T-P-R832, a protein convertase consensus, to generate a NH2 terminus located 36 amino acids upstream of the NH2 terminal found in vivo at Ala869. This intermediate is ultimately cleaved to the mature LPH form by other proteases including the pancreatic ones. These data demonstrate that human pro-LPH, like the rabbit enzyme, is processed to the mature enzyme by furin or furin-like enzymes through at least an intermediate form that has, however, an apparent mass close to that of the mature enzyme.

Functional diversity and interactions between the repeat domains of rat intestinal lactase

Lactase-phlorizin hydrolase (LPH), a major digestive enzyme in the small intestine of newborns, is synthesized as a high-molecular-mass precursor comprising four tandemly repeated domains. Proteolytic cleavage of the precursor liberates the pro segment (LPHalpha) corresponding to domains I and II and devoid of known enzymic function. The mature enzyme (LPHbeta) comprises domains III and IV and is anchored in the brush border membrane via a C-terminal hydrophobic segment. To analyse the roles of the different domains of LPHalpha and LPHbeta, and the interactions between them, we have engineered a series of modified derivatives of the rat LPH precursor. These were expressed in cultured cells under the control of a cytomegalovirus promoter. The results show that recombinant LPHbeta harbouring both domains III and IV produces lactase activity. Neither domain III nor IV is alone sufficient to generate active enzyme, although the corresponding proteins are transport-competent. Tandem duplication of domains III or IV did not restore lactase activity, demonstrating the separate roles of both domains within LPHbeta. Further, the development of lactase activity did not require LPHalpha; however, LPHalpha potentiated the production of active LPHbeta but the individual LPHalpha subdomains I and II were unable to do so. Lactase activity and targeting required the C-terminal transmembrane anchor of LPH; this requirement was terminal transmembrane anchor or LPH; this requirement was not satisfied by the signal/anchor region of another digestive enzyme: sucrase-isomaltase. On the basis of this study we suggest that multiple levels of intramolecular interactions occur within the LPH precursor to produce the mature enzyme, and that the repeat domains of the precursor have distinct and specific functions in protein processing, substrate recognition and catalysis. We propose a functional model of LPHbeta in which substrate is channelled from an entry point located within domain II to the active site located in domain IV.

Human lactase-phlorizin hydrolase is not processed by furin, PC1/PC3, PC2, PACE4 and PC5/PC6A of the family of subtilisin-like proprotein processing proteases

Human lactase-phlorizin hydrolase (LPH, EC 3.2.1.23/62) is synthesized as a single-chain precursor glycoprotein (pro-LPH) with a relative molecular mass of just over 200 kDa. Maturation to the mature enzyme (m-LPH, 160 kDa) occurs after passage of pro-LPH through the Golgi complex and involves the proteolytic removal of a 849 amino acid propeptide. The role of this propeptide as well as its removal is not fully understood and the proteolytic enzyme or enzymes involved are unknown. We studied the potential role of five different members of the family of subtilisin-like proprotein processing proteases in the maturation process of human LPH using a vaccinia virus based coexpression system in pig kidney PK(15) cells. Infected/transfected PK(15) cells expressed full-length pro-LPH but no maturation to m-LPH was observed. Coexpression of human pro-LPH with human furin, human PC1/PC3, human PC2, human PACE4 and mouse PC6A in PK(15) cells did not result in maturation of the enzyme. Cleavage and secretion of von Willebrand factor precursor (pro-vWF) was used as a positive control. None of the five proprotein processing proteases tested were capable of cleaving human pro-LPH, strongly suggesting that they are not involved in the maturation of this enzyme.

Maturation of human intestinal lactase-phlorizin hydrolase: generation of the brush border form of the enzyme involves at least two proteolytic cleavage steps

Human lactase-phlorizin hydrolase (LPH), a brush border membrane hydrolase of the small intestine, is synthesized as a precursor molecule that undergoes proteolytic cleavage to yield mature LPH (LPHbeta) by a trypsin-like protease (Naim et al., 1987, 1991). Arg868-Ala869 has been previously proposed to be the putative cleavage site for this processing step. Site-directed mutagenesis of this monobasic site does not lead to the generation of an uncleaved proLPH species, which strongly suggests the existence of an additional cleavage site. Further analyses of LPH synthesized in different cell lines lend support to this hypothesis. Biosynthetic labeling of human intestinal biopsy samples in the presence of trypsin reveals an LPHbeta species that is slightly smaller than the intracellularly cleaved molecule. When the proLPH molecule is screened for potential cleavage sites, two dibasic pairs are revealed upstream of the N-terminal end of brush border LPH at Lys851-Arg852 and Arg830-Lys831. Treatment of proLPH with trypsin for different periods of time supports the idea of at least two cleavage steps, whereby Arg868-Ala869 represents the final cleavage site that generates LPHbeta. We propose that the initial cleavage of proLPH takes place intracellularly at a site further away from Arg868-Ala869, to generate LPHbeta initial; LPHbeta is subsequently cleaved extracellularly in the gut lumen, presumably by trypsin, at Arg868-Ala869 to mature brush border LPH (LPHbeta initial).

Furin, PC1/3, and/or PC6A process rabbit, but not human, pro-lactase-phlorizin hydrolase to the 180-kDa intermediate

Small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a large precursor (prepro-LPH) of 1926 amino acids. In the endoplasmic reticulum, prepro-LPH is split by signal protease. The resulting pro-LPH is cut to mature LPH directly (human) or via a 180-kDa intermediate (rabbit), most likely in the trans-Golgi network or in a later compartment. Antibodies directed against different regions of rabbit pro-LPH locate the cleavage site resulting in the 180-kDa intermediate between amino acid residues 79 and 286. This stretch contains the two sequences -Arg-Cys-Tyr-Arg114 approximately -Arg-Ala-Ser-Arg191 approximately, which are potential cleavage sites for subtilisin-like proprotein convertases. These sites are not conserved in human pro-LPH. By coexpression in COS 7 cells of rabbit prepro-LPH and proprotein convertases (PC 1/3, PC2, PC6A, PC6B, furin), we show that furin, PC 1/3, and PC6A generate a processing intermediate that is immunologically indistinguishable from the one observed in vivo. Furin, PC 1/3, and PC6A are all expressed in the small intestine as shown by a polymerase chain reaction-based approach and, more specifically, in enterocytes, as shown by in situ hybridization. These results suggest that furin, PC 1/3, and/or PC6A are responsible for the in vivo processing of rabbit pro-LPH to the 180-kDa intermediate.

Folding of human intestinal lactase-phlorizin hydrolase

The folding of human intestinal prolactase-phlorizin hydrolase (pro-LPH) has been analyzed in a cell-free transcription/translation system. In the presence of the thiol oxidant GSSG, disulfide bond formation in pro-LPH can be promoted concomitant with the binding of the molecule to a conformation-specific monoclonal anti-LPH antibody. Under these conditions, pro-LPH does not bind to the molecular chaperone BiP. In the absence of GSSG, on the other hand, pro-LPH does not bind to the monoclonal anti-LPH antibody, but can be immunoprecipitated with a polyclonal antibody that is directed against a denatured form of the enzyme. In this case, interaction of pro-LPH with immunoglobulin heavy chain binding protein can be discerned. The results demonstrate the existence of intramolecular disulfide bonds that are essential for the promotion of pro-LPH to a native conformation. Furthermore, BiP is involved in the folding events of pro-LPH.

Lactase and sucrase-isomaltase gene expression during Caco-2 cell differentiation

The Caco-2 cell line is derived from a human colon adenocarcinoma and differentiates in vitro into small-intestinal enterocyte-like cells, expressing the hydrolases lactase and sucrase-isomaltase. We cultured Caco-2 cells on permeable supports from 0 to 37 days after plating to study endogenous lactase and sucrase-isomaltase gene expression in relation to cell differentiation. Profiles of lactase and sucrase-isomaltase mRNA, protein and enzyme activity were analysed on a per-cell basis, using immunocytochemistry, RNase protection assays, metabolic polypeptide labelling and enzyme activity assays. Tight-junction formation was complete 6 days after plating. Immunocytochemistry of Caco-2 cross-sections showed lactase and sucrase-isomaltase predominantly in the microvillar membrane of polarized cells. mRNA, protein and enzyme activity of lactase appeared consecutively, reaching maximum levels 8-11 days after plating. Whereas lactase mRNA and protein biosynthesis showed a sharp decline after peak levels, lactase activity remained high until 37 days after plating. In contrast, mRNA and protein biosynthesis and activity of sucrase-isomaltase peaked successively 11-21 days after plating, and exhibited comparable levels throughout the entire experiment. The following conclusions were reached. (1) In Caco-2 cells, biosynthesis of lactase and sucrase-isomaltase is regulated by the amount of their mRNAs, indicating transcriptional control. (2) Sucrase-isomaltase activity is most probably transcriptionally controlled at all time points. (3) In contrast, lactase activity is initially regulated by its level of biosynthesis. After its peak at 8 days, the slow decline in activity compared with its biosynthesis indicates high stability. (4) Different mRNA profiles for lactase and sucrase-isomaltase indicate different mechanisms of transcriptional regulation of these genes.

Developmental changes in intestinal glycosylation: nutrition-dependent multi-factor regulation of the fucosylation pathway at weaning time

Developmental changes in the fucoglycoproteins of the intestinal brush-border membranes were determined by lectin affinoblotting after electrophoresis. Whereas only two alpha(1-6)-fucoglycoproteins were detected in brush-border membranes from suckling rats, a large number of N-fucoglycoproteins with alpha(1-2)- and/or alpha(1-6)-linked fucose residues were detected in rat membranes after weaning. Dietary manipulations at weaning time were used to investigate the effect of nutritional factors in the development of fucosylation in the small intestine of prolonged-nursed rats fed with milk (a high-fat, low-carbohydrate diet) compared to rats weaned normally with a standard high-carbohydrate diet. The fucose content of the mucosa glycoproteins was lower in 22-day-old prolonged-nursed rats than in 22-day-old rats weaned normally with the standard diet. The appearance of fucoglycoproteins in the brush-border membranes, which was delayed by prolonged nursing, was accompanied by a concomitant delay in the increase of intestinal fucosyl-transferase activity and in the decrease of GDP-fucose substrate breakdown. The developmental decrease in the activity of the inhibitory protein which regulates the fucosyl-transferase activity was also delayed by prolonged nursing. The intestinal fucosylation of brush-border membrane glycoproteins (which include many digestive enzymes) displayed ontogenic changes on which were superimposed dietary influences at the time of weaning. The complete maturation of the brush-border membrane glycoproteins, and particularly their terminal fucosylation, is a developmental event which thus seems to be strongly influenced by the manipulation of nutritional factors during the weaning period.

Intestinal brush border glycohydrolases: structure, function, and development

The hydrolytic enzymes of the intestinal brush border membrane are essential for the degradation of nutrients to absorbable units. Particularly, the brush border glycohydrolases are responsible for the degradation of di- and oligosaccharides into monosaccharides, and are thus crucial for the energy-intake of humans and other mammals. This review will critically discuss all that is known in the literature about intestinal brush border glycohydrolases. First, we will assess the importance of these enzymes in degradation of dietary carbohydrates. Then, we will closely examine the relevant features of the intestinal epithelium which harbors these glycohydrolases. Each of the glycohydrolytic brush border enzymes will be reviewed with respect to structure, biosynthesis, substrate specificity, hydrolytic mechanism, gene regulation and developmental expression. Finally, intestinal disorders will be discussed that affect the expression of the brush border glycohydrolases. The clinical consequences of these enzyme deficiency disorders will be discussed. Concomitantly, these disorders may provide us with important details regarding the functions and gene expression of these enzymes under specific (pathogenic) circumstances.

Processing and transport of human small intestinal lactase-phlorizin hydrolase (LPH). Role of N-linked oligosaccharide modification

The effect of glycosylation on the intracellular transport of human intestinal lactase-phlorizin hydrolase (LPH) was investigated by biosynthetic labeling of biopsy samples in the presence or absence of glycosidase inhibitors. In the presence of deoxynojirimycin (dNM) and deoxymannojirimycin (dMM), endo H sensitive LPH glycoforms of M(r) = 135,000 in both cases were produced (LPHdNM and LPHdMM). The LPH glycoform generated in the presence of swainsonine had an apparent molecular mass of 141,000 (LPHSwa) and was partially sensitive to endo H. By contrast to unmodified mature LPH (LPHm, M(r) = 160,000), these glycoforms are either not O-glycosylated (LPHdNM and LPHdMM) or partially O-glycosylated (LPHSwa) indicating that processing of N-linked carbohydrates has direct effects on the O-glycosylation of pro-LPH. Analysis of transport kinetics of the various glycoforms strongly suggested that carbohydrate modification does not affect the transport of pro-LPH from the cis-Golgi to the cell surface, but could be rate limiting at the level of the ER.

Analysis of lactase processing in rabbit

The proteolytic processing of rabbit intestinal lactase-phlorizin-hydrolase (LPH) was studied by pulse-chase and continuous labeling experiments in organ culture from 15-day-old rabbits in the presence of glycosylation and processing inhibitors. Monensin and brefeldin A inhibited the two proteolytic cleavages of the precursor indicating that they are post-Golgi events as previously reported for the unique cleavage of LPH in man. The inhibition was not related to a concomitant alteration glycosylation; in fact, if trimming was blocked by MDNM the abnormal glycosylated precursor was proteolytically processed normally. Finally the use of the anti-microtubular drug colchicine strongly inhibited both cleavages and caused accumulation of the complex-glycosylated precursor form the brush border fraction indicating that proteolytic events depend on intact microtubule (transport).

Proteolytic processing of human intestinal lactase-phlorizin hydrolase precursor is not a prerequisite for correct sorting in Madin Darby canine kidney (MDCK) cells

Maturation of lactase-phlorizin hydrolase (LPH) (EC 3.2.1.23-62) requires proteolytic processing of precursor (pro-LPH) to mature microvillus membrane enzyme (m-LPH). Subcellular site and function of this processing are unknown. We studied the processing and sorting of human LPH expressed permanently in MDCK cells. LPH was inserted into the apical membrane and small amounts were found basolateral. Of the LPH immunoprecipitated from the apical membrane, 42% was in the mature, i.e. proteoytically processed form; on the basolateral membrane it was 20%. Thus, LPH-processing occurs after sorting and is not necessary for surface expression.

Maturation of human lactase-phlorizin hydrolase. Proteolytic cleavage of precursor occurs after passage through the Golgi complex

Maturation of human intestinal lactase-phlorizin hydrolase (LPH) requires that a precursor (pro-LPH) be proteolytically processed to the mature microvillus membrane enzyme (m-LPH). The subcellular site of this processing is unknown. Using low-temperature experiments and brefeldin A (BFA), intracellular transport was blocked in intestinal epithelial cells. In Caco-2 cells incubated at 18 degrees C, pro-LPH was complex-glycosylated but not cleaved, while at 20 degrees C small amounts of proteolytically processed LPH were observed. These data exclude a pre-Golgi proteolytic event. BFA completely blocked proteolytic maturation of LPH and lead to an aberrant form of pro-LPH in both Caco-2 cells and intestinal explants. Therefore, proteolytic processing of LPH is a post-Golgi event, occurring either in the trans-Golgi network, transport vesicles, or after insertion of pro-LPH into the microvillus membrane.