Protein domains implicated in intracellular transport and sorting of lactase-phlorizin hydrolase

Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives

Congenital lactase deficiency (CLD) is a severe autosomal recessive genetic disorder that affects the functional capacity of the intestinal protein lactase-phlorizin hydrolase (LPH). This disorder is diagnosed already during the first few days of the newborn’s life due to the inability to digest lactose, the main carbohydrate in mammalian milk. The symptoms are similar to those in other carbohydrate malabsorption disorders, such as congenital sucrase-isomaltase deficiency, and include severe osmotic watery diarrhea. CLD is associated with mutations in the translated region of the LPH gene that elicit loss-of-function of LPH. The mutations occur in a homozygote or compound heterozygote pattern of inheritance and comprise missense mutations as well as mutations that lead to complete or partial truncations of crucial domains in LPH, such as those linked to the folding and transport-competence of LPH and to the catalytic domains. Nevertheless, the identification of the mutations in CLD is not paralleled by detailed genotype/protein phenotype analyses that would help unravel potential pathomechanisms underlying this severe disease. Here, we review the current knowledge of CLD mutations and discuss their potential impact on the structural and biosynthetic features of LPH. We also address the question of whether heterozygote carriers can be symptomatic for CLD and whether genetic testing is needed in view of the severity of the disease.

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.

The Diverse Forms of Lactose Intolerance and the Putative Linkage to Several Cancers

Lactase-phlorizin hydrolase (LPH) is a membrane glycoprotein and the only β-galactosidase of the brush border membrane of the intestinal epithelium. Besides active transcription, expression of the active LPH requires different maturation steps of the polypeptide through the secretory pathway, including N- and O-glycosylation, dimerization and proteolytic cleavage steps. The inability to digest lactose due to insufficient lactase activity results in gastrointestinal symptoms known as lactose intolerance. In this review, we will concentrate on the structural and functional features of LPH protein and summarize the cellular and molecular mechanism required for its maturation and trafficking. Then, different types of lactose intolerance are discussed, and the molecular aspects of lactase persistence/non-persistence phenotypes are investigated. Finally, we will review the literature focusing on the lactase persistence/non-persistence populations as a comparative model in order to determine the protective or adverse effects of milk and dairy foods on the incidence of colorectal, ovarian and prostate cancers.

Structural hierarchy of regulatory elements in the folding and transport of an intestinal multidomain protein

Human intestinal lactase-phlorizin hydrolase, LPH, encompasses four homologous domains, which presumably have evolved from two subsequent duplications of one ancestral gene. The profragment, LPHalpha, comprises homologous domains I and II and functions as an intramolecular chaperone in the context of the brush-border LPHbeta region of LPH. Here, we analyze the inter-relationship between homologous domains III and IV of LPHbeta and their implication in the overall structure, function, and trafficking of LPH. In silico analyses revealed potential domain boundaries for these domains as a basis for loop-out mutagenesis and construction of deletion or individual domain forms of LPH. Removal of domain IV, which contains lactase, results in a diminished phlorizin hydrolase activity, lack of dimerization in the endoplasmic reticulum (ER), but accelerated transport kinetics from the ER to the Golgi apparatus. By contrast, deletion of domain III, which harbors phlorizin hydrolase, generates a malfolded protein that is blocked in the ER. Interestingly, homologous domain III is transport-competent per se and sorted to the apical membrane in polarized Madin-Darby canine kidney cells. Nevertheless, it neither dimerizes nor acquires complete phlorizin hydrolase activity. Our data present a hierarchical model of LPH in which the homologous domain III constitutes (i) a fully autonomous core domain within LPH and (ii) another intramolecular chaperone besides the profragment LPHalpha. Nevertheless, the regulation of the trafficking kinetics and activity of domain III and entire LPH including elevation of the enzymatic activities require the correct dimerization of LPH in the ER, an event that is accomplished by the non-autonomous domain IV.

An NSP4-dependant mechanism by which rotavirus impairs lactase enzymatic activity in brush border of human enterocyte-like Caco-2 cells

Lactase-phlorizin hydrolase (LPH, EC 3.2.1.23-62) is a brush border membrane (BBM)-associated enzyme in intestinal cells that hydrolyse lactose, the most important sugar in milk. Impairing in lactase activity during rotavirus infection has been described in diseased infants but the mechanism by which the functional lesion occurs remains unknown. We undertook a study to elucidate whether rotavirus impairs the lactase enzymatic activity in BBM of human enterocyte cells. In this study we use cultured human intestinal fully differentiated enterocyte-like Caco-2 cells to demonstrate how the lactase enzymatic activity at BBM is significantly decreased in rhesus monkey rotavirus (RRV)-infected cells. We found that the decrease in enzyme activity is not dependent of the Ca(2+)- and cAMP-dependent signalling events triggered by the virus. The LPH biosynthesis, stability, and expression of the protein at the BBM of infected cells were not modified. We provide evidence that in RRV-infected cells the kinetic of lactase enzymatic activity present at the BBM was modified. Both BBM(control) and BBM(RRV) have identical K(m) values, but hydrolyse the substrate at different rates. Thus, the BBM(RRV) exhibits almost a 1.5-fold decreased V(max) than that of BBM(control) and is therefore enzymatically less active than the latter. Our study demonstrate conclusively that the impairment of lactase enzymatic activity at the BBM of the enterocyte-like Caco-2 cells observed during rotavirus infection results from an inhibitory action of the secreted non-structural rotavirus protein NSP4.

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.

The prosequence of human lactase-phlorizin hydrolase modulates the folding of the mature enzyme

The efficient transport of proteins along the secretory pathway requires that the polypeptide adopts a stably folded conformation to egress the endoplasmic reticulum (ER). The transport-competent precursor of the brush border enzyme LPH, pro-LPH, undergoes an intracellular cleavage process in the trans-Golgi network between Arg(734) and Leu(735) to yield LPH beta(initial). The role of the prodomain comprising the N-terminally located 734 amino acids of pro-LPH, LPH alpha, in the folding events of LPH beta(initial) has been analyzed by the individual expression of both forms in COS-1 cells. Following synthesis at 37 degrees C LPH beta(initial) acquires a misfolded and enzymatically inactive conformation that is degraded by trypsin. A temperature shift to 20 degrees C generates a stable, trypsin-resistant, and enzymatically active LPH beta(initial) indicating that the individual expression of LPH beta(initial) results in a temperature-sensitive conformation. This form interacts at non-permissive temperatures sequentially with the ER chaperones immunoglobulin-binding protein and calnexin resulting in an ER retention. The LPH alpha prodomain resides in the ER when individually expressed. It reveals compact structural features that are stabilized by disulfide bridges. LPH alpha and LPH beta(initial) readily interact with each other upon coexpression, and this interaction appears to trigger the formation of a trypsin-resistant, correctly folded, enzymatically active, and transport-competent LPH beta(initial) polypeptide. These data clearly demonstrate that the proregion of pro-LPH is an intramolecular chaperone that is critically essential in facilitating the folding of the intermediate form LPH beta(initial) in the context of the pro-LPH polypeptide.

Mucin-like domain of enteropeptidase directs apical targeting in Madin-Darby canine kidney cells

Enteropeptidase, a type II transmembrane protein of the enterocyte brush border, is sorted directly to the apical membrane of Madin-Darby canine kidney II cells. Apical targeting appears to be mediated by an N-terminal segment that contains a 27-amino acid residue O-glycosylated mucin-like domain consisting of two short mucin-like repeats, A and B. Targeting signals within these repeats were characterized by using green fluorescent protein (GFP) as a reporter. Constructs with a cleavable signal peptide and both repeats A and B were secreted apically. Similar constructs lacking mucin repeats were secreted randomly. Either repeat A or B was sufficient to direct apical targeting of GFP. O-linked oligosaccharides alone were not sufficient for targeting because fusion to a different O-glycosylated motif did not alter the random secretion of GFP, and several constructs with mutations in either repeat A or B were O-glycosylated and secreted randomly. In addition, repeat B appears to contain an apical targeting signal that functions in the absence of glycosylation. Density gradient centrifugation indicated that, unlike several other apically targeted membrane and soluble proteins, apical sorting of mucin-GFP chimeric proteins does not appear to utilize lipid rafts.

Additional N-glycosylation and its impact on the folding of intestinal lactase-phlorizin hydrolase

Lactase-phlorizin hydrolase (LPH) is a membrane bound intestinal hydrolase, with an extracellular domain comprising 4 homologous regions. LPH is synthesized as a large polypeptide precursor, pro-LPH, that undergoes several intra- and extracellular proteolytic steps to generate the final brush-border membrane form LPHbeta(final). Pro-LPH is associated through homologous domain IV with the membrane through a transmembrane domain. A truncation of 236 amino acids at the COOH terminus of domain IV (denoted LAC236) does not significantly influence the transport competence of the generated mutant LPH1646MACT (Panzer, P., Preuss, U., Joberty, G., and Naim, H. Y. (1998) J. Biol. Chem. 273, 13861-13869), strongly suggesting that LAC236 is an autonomously folded domain that links the ectodomain with the transmembrane region. Here, we examine this hypothesis by engineering several N-linked glycosylation sites into LAC236. Transient expression of the cDNA constructs in COS-1 cells confirm glycosylation of the introduced sites. The N-glycosyl pro-LPH mutants are transported to the Golgi apparatus at substantially reduced rates as compared with wild-type pro-LPH. Alterations in LAC236 appear to sterically hinder the generation of stable dimeric trypsin-resistant pro-LPH forms. Individual expression of chimeras containing LAC236, the transmembrane domain and cytoplasmic tail of pro-LPH and GFP as a reporter gene (denoted LAC236-GFP) lends strong support to this view: while LAC236-GFP is capable of forming dimers per se, its N-glycosyl variants are not. The data strongly suggest that the LAC236 is implicated in the dimerization process of pro-LPH, most likely by nucleating the association of the ectodomains of the enzyme.

Requirement of a leucine residue for (apical) membrane expression of type IIb NaPi cotransporters

Type II NaPi cotransporters mediate epithelial phosphate (P(i)) reabsorption. In mammals the type IIb protein is expressed in the small intestinal apical membrane and other epithelia; it is not expressed in the renal proximal tubule where we find the type IIa isoform. To look for molecular determinant(s) involved in apical expression of type IIb cotransporters, we have made deletion mutations within the C-terminal tails of mouse IIb (mIIb) and human IIb (hIIb) transporter proteins. The constructs were fused to the enhanced green fluorescent protein and transiently transfected into intestinal CaCo2-cells. Both mIIb and hIIb were located exclusively in the apical membrane of the cells. For mIIb, the removal of a cysteine cluster or the last three amino acids (TVF) had no effect on the location of the protein. However, truncation at the level of the conserved L691/689 prevented the apical membrane expression of both mIIb and hIIb, respectively, and the mutated proteins were located in endosomal and lysosomal structures. A similar expression pattern of the mIIb and hIIb constructs was found in renal proximal tubular opossum kidney cells. Our data suggest that L691/689 is involved in mechanisms leading to an apical expression of type IIb NaPi cotransporters.

Species differences in the sites of cleavage of pro-lactase to lactase supports lack of selective pressure

The pro-sequences in pro-lactase-phlorizin hydrolase (LPH) are needed for lactase to proceed past the ER, but are irrelevant as to the enzymatic activities. Hence, in all species removal of the pro- sequences (or most of them) must take place after the ER. Contrary to this, the details of the removal of these pro-sequences are to be expected to differ in the various species, since they are not subjected to selective pressure. Using site-directed mutagenesis we investigated processing in rabbit. The first cleavage occurs by furin (or furin-like PCs) and takes place at R-A-A-R(349) in the pro-sequence, generating the known 180 kDa intermediate. Replacing R(349) by Q results in a mutant which is not cleaved but nevertheless transported to the cell surface as demonstrated by immunofluorescence. Further processing of either the 180 kDa intermediate or the mutant is not directly mediated by furin-like PCs, but involves (also) other proteases. These results demonstrate that formation of the 180 kDa intermediate, consistently found only in rabbits, but not in man, is not essential for lactase transport: in all likelihood lack of selective pressure has led to species-specific processing of pro-LPH.

Hierarchy of sorting signals in chimeras of intestinal lactase-phlorizin hydrolase and the influenza virus hemagglutinin

Lactase-phlorizin hydrolase (LPH) is an apical protein in intestinal cells. The location of sorting signals in LPH was investigated by preparing a series of mutants that lacked the LPH cytoplasmic domain or had the cytoplasmic domain of LPH replaced by sequences that comprised basolateral targeting signals and overlapping internalization signals of various potency. These signals are mutants of the cytoplasmic domain of the influenza hemagglutinin (HA), which have been shown to be dominant in targeting HA to the basolateral membrane. The LPH-HA chimeras were expressed in Madin-Darby canine kidney (MDCK) and colon carcinoma (Caco-2) cells, and their transport to the cell surface was analyzed. All of the LPH mutants were targeted correctly to the apical membrane. Furthermore, the LPH-HA chimeras were internalized, indicating that the HA tails were available to interact with the cytoplasmic components of clathrin-coated pits. The introduction of a strong basolateral sorting signal into LPH was not sufficient to override the strong apical signals of the LPH external domain or transmembrane domains. These results show that basolateral sorting signals are not always dominant over apical sorting signals in proteins that contain each and suggest that sorting of basolateral from apical proteins occurs within a common compartment where competition for sorting signals can occur.

Production of low-lactose milk by ectopic expression of intestinal lactase in the mouse mammary gland

We have investigated, in mice, an in vivo method for producing low-lactose milk, based on the creation of transgenic animals carrying a hybrid gene in which the intestinal lactase-phlorizin hydrolase cDNA was placed under the control of the mammary-specific alpha-lactalbumin promoter. Transgenic females expressed lactase protein and activity during lactation at the apical side of mammary alveolar cells. Active lactase was also secreted into milk, anchored in the outer membrane of fat globules. Lactase synthesis in the mammary gland caused a significant decrease in milk lactose (50-85%) without obvious changes in fat and protein concentrations. Sucklings nourished with low-lactose milk developed normally. Hence, these data validate the use of transgenic animals expressing lactase in the mammary gland to produce low-lactose milk in vivo, and they demonstrate that the secretion of an intestinal digestive enzyme into milk can selectively modify its composition.