Molecular basis of aberrant apical protein transport in an intestinal enzyme disorder

Genetic and acquired sucrase-isomaltase deficiency: A clinical review

Genetic sucrase-isomaltase deficiency (GSID) is an inherited deficiency in the ability to digest sucrose and potentially starch due to mutations in the sucrase-isomaltase (SI) gene. Congenital sucrase-isomaltase deficiency is historically considered to be a rare condition affecting infants with chronic diarrhea as exposure to dietary sucrose begins. Growing evidence suggests that individuals with SI variants may present later in life, with symptoms overlapping with those of irritable bowel syndrome. The presence of SI genetic variants may, either alone or in combination, affect enzyme activity and lead to symptoms of different severity. As such, a more appropriate term for this inherited condition is GSID, with a recognition of a spectrum of severity and onset of presentation. Currently, disaccharidase assay on duodenal mucosal tissue homogenates is the gold standard in diagnosing SI deficiency. A deficiency in the SI enzyme can be present at birth (genetic) or acquired later, often in association with damage to the enteric brush-border membrane. Other noninvasive diagnostic alternatives such as sucrose breath tests may be useful but require further validation. Management of GSID is based on sucrose and potentially starch restriction tailored to the individual patients' tolerance and symptoms. As this approach may be challenging, additional treatment with commercially available sacrosidase is available. However, some patients may require continued starch restriction. Further research is needed to clarify the true prevalence of SI deficiency, the pathobiology of single SI heterozygous mutations, and to define optimal diagnostic and treatment algorithms in the pediatric population.

Genetic Loss of Sucrase-Isomaltase Function: Mechanisms, Implications, and Future Perspectives

Genetic variants causing loss of sucrase-isomaltase (SI) function result in malabsorption of sucrose and starch components and the condition congenital sucrase-isomaltase deficiency (CSID). The identified genetic variants causing CSID are very rare in all surveyed populations around the globe, except the Arctic-specific c.273_274delAG loss-of-function (LoF) variant, which is common in the Greenlandic Inuit and other Arctic populations. In these populations, it is, therefore, possible to study people with loss of SI function in an unbiased way to elucidate the physiological function of SI, and investigate both short-term and long-term health effects of reduced small intestinal digestion of sucrose and starch. Importantly, a recent study of the LoF variant in Greenlanders reported that adult homozygous carriers have a markedly healthier metabolic profile. These findings indicate that SI inhibition could potentially improve metabolic health also in individuals not carrying the LoF variant, which is of great interest considering the massive number of individuals with obesity and type 2 diabetes worldwide. Therefore, the objectives of this review, are 1) to describe the biological role of SI, 2) to describe the metabolic impact of the Arctic SI LoF variant, 3) to reflect on potential mechanisms linking reduced SI function to metabolic health, and 4) to discuss what knowledge is necessary to properly evaluate whether SI inhibition is a potential therapeutic target for improving cardiometabolic health.

Two Novel Mutations in the SI Gene Associated With Congenital Sucrase-Isomaltase Deficiency: A Case Report in China

Background: Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive inherited disease that leads to the maldigestion of disaccharides and is associated with mutation of the sucrase-isomaltase (SI) gene. Cases of CSID are not very prevalent in China or worldwide but are gradually being identified and reported. Case Presentation: We report a case involving a 14-month-old male who presented with failure to thrive that had begun after food diversification and was admitted for chronic diarrhea. We used a whole-exome sequencing (WES) approach to identify mutations in this patient's genome. WES revealed two novel heterozygous mutations in the SI gene, c.2626C > T (p.Q876^*) and c.2872C > T (p.R958C), which were confirmed by Sanger DNA sequencing. With a strict sucrose- and starch-restricted diet, the patient's diarrhea was resolved, and he began to gain weight. Conclusions: We report a case of novel variants in the SI gene that caused CSID. This report provides valuable information for the clinical field, especially in China.

Differential Effects of Sucrase-Isomaltase Mutants on Its Trafficking and Function in Irritable Bowel Syndrome: Similarities to Congenital Sucrase-Isomaltase Deficiency

Congenital sucrase-isomaltase deficiency (CSID) is a rare metabolic intestinal disorder with reduced or absent activity levels of sucrase-isomaltase (SI). Interestingly, the main symptoms of CSID overlap with those in irritable bowel syndrome (IBS), a common functional gastrointestinal disorder with unknown etiology. Recent advances in genetic screening of IBS patients have revealed rare SI gene variants that are associated with IBS. Here, we investigated the biochemical, cellular and functional phenotypes of several of these variants. The data demonstrate that the SI mutants can be categorized into three groups including immature, mature but slowly transported, and finally mature and properly transported but with reduced enzymatic activity. We also identified SI mutant phenotypes that are deficient but generally not as severe as those characterized in CSID patients. The variable effects on the trafficking and function of the mutations analyzed in this study support the view that both CSID and IBS are heterogeneous disorders, the severity of which is likely related to the biochemical phenotypes of the SI mutants as well as the environment and diet of patients. Our study underlines the necessity to screen for SI mutations in IBS patients and to consider enzyme replacement therapy as an appropriate therapy as in CSID.

Hypomorphic SI genetic variants are associated with childhood chronic loose stools

OBJECTIVE

The SI gene encodes the sucrase-isomaltase enzyme, a disaccharidase expressed in the intestinal brush border. Hypomorphic SI variants cause recessive congenital sucrase-isomaltase deficiency (CSID) and related gastrointestinal (GI) symptoms. Among children presenting with chronic, idiopathic loose stools, we assessed the prevalence of CSID-associated SI variants relative to the general population and the relative GI symptom burden associated with SI genotype within the study population.

METHODS

A prospective study conducted at 18 centers enrolled 308 non-Hispanic white children ≤18 years old who were experiencing chronic, idiopathic, loose stools at least once per week for >4 weeks. Data on demographics, GI symptoms, and genotyping for 37 SI hypomorphic variants were collected. Race/ethnicity-matched SI data from the Exome Aggregation Consortium (ExAC) database was used as the general population reference.

RESULTS

Compared with the general population, the cumulative prevalence of hypomorphic SI variants was significantly higher in the study population (4.5% vs. 1.3%, P < .01; OR = 3.5 [95% CI: 6.1, 2.0]). Within the study population, children with a hypomorphic SI variant had a more severe GI symptom burden than those without, including: more frequent episodes of loose stools (P < .01), higher overall stool frequency (P < .01), looser stool form (P = .01) and increased flatulence (P = .02).

CONCLUSION

Non-Hispanic white children with chronic idiopathic loose stools have a higher prevalence of CSID-associated hypomorphic SI variants than the general population. The GI symptom burden was greater among the study subjects with a hypomorphic SI variant than those without hypomorphic SI variants.

Heterozygotes Are a Potential New Entity among Homozygotes and Compound Heterozygotes in Congenital Sucrase-Isomaltase Deficiency

Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disorder of carbohydrate maldigestion and malabsorption caused by mutations in the sucrase-isomaltase (SI) gene. SI, together with maltase-glucoamylase (MGAM), belongs to the enzyme family of disaccharidases required for breakdown of α-glycosidic linkages in the small intestine. The effects of homozygote and compound heterozygote inheritance trait of SI mutations in CSID patients have been well described in former studies. Here we propose the inclusion of heterozygote mutation carriers as a new entity in CSID, possibly presenting with milder symptoms. The hypothesis is supported by recent observations of heterozygote mutation carriers among patients suffering from CSID or patients diagnosed with functional gastrointestinal disorders. Recent studies implicate significant phenotypic heterogeneity depending on the character of the mutation and call for more research regarding the correlation of genetics, function at the cellular and molecular level and clinical presentation. The increased importance of SI gene variants in irritable bowel syndrome (IBS) or other functional gastrointestinal disorders FGIDs and their available symptom relief diets like fermentable oligo-, di-, mono-saccharides and polyols FODMAPs suggest that the heterozygote mutants may affect the disease development and treatment.

Posttranslational Processing and Function of Mucosal Maltases

The final step of carbohydrate digestion in the intestine is performed by 2 major α-glucosidases of the intestinal mucosa, sucrase-isomaltase (SI) and maltase-glucoamylase. Both of these enzymes are type II membrane glycoproteins, which share a significant level of homology in gene and protein structures and yet have differences in the posttranslational processing, substrate specificity and functional capacity. Insufficient activity of these disaccharidases particularly SI as a result of genetic mutations or secondary intestinal pathologies is associated with carbohydrate maldigestion and gastrointestinal intolerances. This review will discuss the maturation profiles of SI and maltase-glucoamylase relative to their functional capacities and deficiencies.

Congenital Sucrase-isomaltase Deficiency: A Novel Compound Heterozygous Mutation Causing Aberrant Protein Localization

OBJECTIVES

Congenital diarrheal disorders is a group of inherited enteropathies presenting in early life and requiring parenteral nutrition. In most cases, genetics may be the key for precise diagnosis. We present an infant girl with chronic congenital diarrhea that resolved after introduction of fructose-based formula but had no identified mutation in the SLC5A1 gene. Using whole exome sequencing (WES) we identified other mutations that better dictated dietary adjustments.

METHODS

WES of the patient and her parents was performed. The analysis focused on recessive model including compound heterozygous mutations. Sanger sequencing was used to validate identified mutations and to screen the patient's newborn sister and grandparents. Expression and localization analysis were performed in the patient's duodenal biopsies using immunohistochemistry.

RESULTS

Using WES we identified a new compound heterozygote mutation in sucrase-isomaltase (SI) gene; a maternal inherited known V577G mutation, and a novel paternal inherited C1531W mutation. Importantly, the newborn offspring carried similar compound heterozygous mutations. Computational predictions suggest that both mutations highly destabilize the protein. SI expression and localization studies determined that the mutated SI protein was not expressed on the brush border membrane in the patient's duodenal biopsies, verifying the diagnosis of congenital sucrase-isomaltase deficiency (CSID).

CONCLUSIONS

The novel compound heterozygote V577G/C1531W SI mutations lead to lack of SI expression in the duodenal brush border, confirming the diagnosis of CSID. These cases of CSID extend the molecular spectrum of this condition, further directing a more adequate dietary intervention for the patient and newborn sibling.

Molecular pathogenicity of novel sucrase-isomaltase mutations found in congenital sucrase-isomaltase deficiency patients

BACKGROUND & AIMS

Congenital sucrase-isomaltase deficiency (CSID) is a genetic disorder associated with mutations in the sucrase-isomaltase (SI) gene. The diagnosis of congenital diarrheal disorders like CSID is difficult due to unspecific symptoms and usually requires invasive biopsy sampling of the intestine. Sequencing of the SI gene and molecular analysis of the resulting potentially pathogenic SI protein variants may facilitate a diagnosis in the future. This study aimed to categorize SI mutations based on their functional consequences.

METHODS

cDNAs encoding 13 SI mutants were expressed in COS-1 cells. The molecular pathogenicity of the resulting SI mutants was defined by analyzing their biosynthesis, cellular localization, structure and enzymatic functions.

RESULTS

Three biosynthetic phenotypes for the novel SI mutations were identified. The first biosynthetic phenotype was defined by mutants that are intracellularly transported in a fashion similar to wild type SI and with normal, but varying, levels of enzymatic activity. The second biosynthetic phenotype was defined by mutants with delayed maturation and trafficking kinetics and reduced activity. The third group of mutants is entirely transport incompetent and functionally inactive.

CONCLUSIONS

The current study unraveled CSID as a multifaceted malabsorption disorder that comprises three major classes of functional and trafficking mutants of SI and established a gradient of mild to severe functional deficits in the enzymatic functions of the enzyme.

GENERAL SIGNIFICANCE

This novel concept and the existence of mild consequences in a number of SI mutants strongly propose that CSID is an underdiagnosed and a more common intestinal disease than currently known.

Natural Selection at the Brush-Border: Adaptations to Carbohydrate Diets in Humans and Other Mammals

Dietary shifts can drive molecular evolution in mammals and a major transition in human history, the agricultural revolution, favored carbohydrate consumption. We investigated the evolutionary history of nine genes encoding brush-border proteins involved in carbohydrate digestion/absorption. Results indicated widespread adaptive evolution in mammals, with several branches experiencing episodic selection, particularly strong in bats. Many positively selected sites map to functional protein regions (e.g., within glucosidase catalytic crevices), with parallel evolution at SI (sucrase-isomaltase) and MGAM (maltase-glucoamylase). In human populations, five genes were targeted by positive selection acting on noncoding variants within regulatory elements. Analysis of ancient DNA samples indicated that most derived alleles were already present in the Paleolithic. Positively selected variants at SLC2A5 (fructose transporter) were an exception and possibly spread following the domestication of specific fruit crops. We conclude that agriculture determined no major selective event at carbohydrate metabolism genes in humans, with implications for susceptibility to metabolic disorders.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

Compound heterozygous mutations affect protein folding and function in patients with congenital sucrase-isomaltase deficiency

BACKGROUND & AIMS

Congenital sucrase-isomaltase (SI) deficiency is an autosomal-recessive intestinal disorder characterized by a drastic reduction or absence of sucrase and isomaltase activities. Previous studies have indicated that single mutations underlie individual phenotypes of the disease. We investigated whether compound heterozygous mutations, observed in some patients, have a role in disease pathogenesis.

METHODS

We introduced mutations into the SI complementary DNA that resulted in the amino acid substitutions V577G and G1073D (heterozygous mutations found in one group of patients) or C1229Y and F1745C (heterozygous mutations found in another group). The mutant genes were expressed transiently, alone or in combination, in COS cells and the effects were assessed at the protein, structural, and subcellular levels.

RESULTS

The mutants SI-V577G, SI-G1073D, and SI-F1745C were misfolded and could not exit the endoplasmic reticulum, whereas SI-C1229Y was transported only to the Golgi apparatus. Co-expression of mutants found on each SI allele in patients did not alter the protein's biosynthetic features or improve its enzymatic activity. Importantly, the mutations C1229Y and F1745C, which lie in the sucrase domains of SI, prevented its targeting to the cell's apical membrane but did not affect protein folding or isomaltase activity.

CONCLUSIONS

Compound heterozygosity is a novel pathogenic mechanism of congenital SI deficiency. The effects of mutations in the sucrase domain of SIC1229Y and SIF1745C indicate the importance of a direct interaction between isomaltase and sucrose and the role of sucrose as an intermolecular chaperone in the intracellular transport of SI.

Multiple post-translational modifications of mouse insulin-like growth factor binding protein-6 expressed in epithelial Madin-Darby canine kidney cells

Insulin-like growth factors (IGFs), IGF receptors and IGF binding proteins (IGFBPs) participate in the regulation of proliferation and differentiation of epithelial cells. Expression of the growth-inhibitory murine IGFBP-6 in epithelial Madin-Darby canine kidney (MDCK) cells followed by 2D analysis revealed the presence of multiple isoforms. Metabolic labelling experiments showed that several IGFBP-6 isoforms are modified by phosphate and sulfate groups. Expression analysis of mutant IGFBP-6 further demonstrated that serine residue 143 is O-glycosylated. Substitution of serine 143 by alanine did slightly reduce the preferential sorting of mIGFBP-6 to the apical site in MDCK cells grown on semipermeable filters. Both the presence of multiple and heterogeneously modified isoforms of murine IGFBP-6 in MDCK cells, and the preferential secretion of non-glycosylated IGFBP-6 mutants to the apical side suggest that the major apical sorting signal is the protein moiety.

Coordinated protein sorting, targeting and distribution in polarized cells

The polarized distribution of functions in polarized cells requires the coordinated interaction of three machineries that modify the basic mechanisms of intracellular protein trafficking and distribution. First, intrinsic protein-sorting signals and cellular decoding machineries regulate protein trafficking to plasma membrane domains; second, intracellular signalling complexes define the plasma membrane domains to which proteins are delivered; and third, proteins that are involved in cell-cell and cell-substrate adhesion orientate the three-dimensional distribution of intracellular signalling complexes and, accordingly, the direction of membrane traffic. The integration of these mechanisms into a complex and dynamic network is crucial for normal tissue function and is often defective in disease states.

An extracellular loop of the human non-gastric H,K-ATPase alpha-subunit is involved in apical plasma membrane polarization

The human non-gastric H,K-ATPase, ATP1AL1, belongs to the gene family of P-type ATPases. Consistent with their physiological roles in ion transport, members of this group, including the Na,KATPase and the gastric and non-gastric H,K-ATPases, are differentially polarized to either the basolateral or apical plasma membrane in epithelial cells. However, their polarized distribution is highly complex and depends on specific sorting signals or motifs which are recognized by the subcellular targeting machinery. For the gastric H,K-ATPase it has been suggested that the 4(th) transmembrane spanning domain (TM4) and its flanking regions induce conformational sorting motifs which direct the ion pump exclusively to the epithelial apical membrane. Here, we show in transfected Madin-Darby canine kidney (MDCK) cells that the related non-gastric H,KATPase, ATP1AL1, does contain similar sorting motifs in close proximity to TM4. A short extracellular loop between TM3 and TM4 is critical for this pump's apical delivery. A single point mutation in the corresponding region redirects ATP1AL1 to the basolateral membrane. In conclusion, our work provides further evidence that the cellular distribution of P-type ATPases is determined by conformational sorting motifs.

Altered Folding, Turnover, and Polarized Sorting Act in Concert to Define a Novel Pathomechanism of Congenital Sucrase-Isomaltase Deficiency

Naturally occurring mutants of membrane and secretory proteins are often associated with the pathogenesis of human diseases. Here, we describe the molecular basis of a novel phenotype of congenital sucrase-isomaltase deficiency (CSID), a disaccharide malabsorption disorder of the human intestine in which several structural features and functional capacities of the brush-border enzyme complex sucrase-isomaltase (SI) are affected. The cDNA encoding SI from a patient with CSID reveals a mutation in the isomaltase subunit of SI that results in the substitution of a cysteine by an arginine at amino acid residue 635 (C635R). When this mutation is introduced into the wild type cDNA of SI a mutant enzyme, SI(C635R), is generated that shows a predominant localization in the endoplasmic reticulum. Nevertheless, a definite localization of SI(C635R) in the Golgi apparatus and at the cell surface could be also observed. Epitope mapping with conformation-specific mAbs protease sensitivity assays, and enzymatic activity measurements demonstrate an altered folding pattern of SI(C635R) that is responsible for a substantially increased turnover rate and an aberrant sorting profile. Thus, SI(C635R) becomes distributed also at the basolateral membrane in contrast to wild type SI. Concomitant with the altered sorting pattern, the partial detergent extractability of wild type SI shifts to a complete detergent solubility with Triton X-100. The mutation has therefore affected an epitope responsible for the apical targeting fidelity of SI. Altogether, the combined effects of the C635R mutation on the turnover rate, function, polarized sorting, and detergent solubility of SI constitute a unique and novel pathomechanism of CSID.

Disaccharide digestion: clinical and molecular aspects

Sugars normally are absorbed in the small intestine. When carbohydrates are malabsorbed, the osmotic load produced by the high amount of low molecular weight sugars and partially digested starches in the small intestine can cause symptoms of intestinal distention, rapid peristalsis, and diarrhea. Colonic bacteria normally metabolize proximally malabsorbed dietary carbohydrate through fermentation to small fatty acids and gases (ie, hydrogen, methane, and carbon dioxide). When present in large amounts, the malabsorbed sugars and starches can be excreted in the stool. Sugar intolerance is the presence of abdominal symptoms related to the proximal or distal malabsorption of dietary carbohydrates. The symptoms consist of meal-related abdominal cramps and distention, increased flatulence, borborygmus, and diarrhea. Infants and young children with carbohydrate malabsorption show more intense symptoms than adults; the passage of undigested carbohydrates through the colon is more rapid and is associated with detectable carbohydrates in copious watery acid stools. Dehydration often follows feeding of the offending sugar. In this review we present the clinical and current molecular aspects of disaccharidase digestion.

Mechanistic and structural analysis of a family 31 alpha-glycosidase and its glycosyl-enzyme intermediate

We have determined the first structure of a family 31 alpha-glycosidase, that of YicI from Escherichia coli, both free and trapped as a 5-fluoroxylopyranosyl-enzyme intermediate via reaction with 5-fluoro-alpha-D-xylopyranosyl fluoride. Our 2.2-A resolution structure shows an intimately associated hexamer with structural elements from several monomers converging at each of the six active sites. Our kinetic and mass spectrometry analyses verified several of the features observed in our structural data, including a covalent linkage from the carboxylate side chain of the identified nucleophile Asp(416) to C-1 of the sugar ring. Structure-based sequence comparison of YicI with the mammalian alpha-glucosidases lysosomal alpha-glucosidase and sucrase-isomaltase predicts a high level of structural similarity and provides a foundation for understanding the various mutations of these enzymes that elicit human disease.

The early stages of the intracellular transport of membrane proteins: clinical and pharmacological implications

Intracellular transport mechanisms ensure that integral membrane proteins are delivered to their correct subcellular compartments. Efficient intracellular transport is a prerequisite for the establishment of both cell architecture and function. In the past decade, transport processes of proteins have also drawn the attention of clinicians and pharmacologists since many diseases have been shown to be caused by transport-deficient proteins. Membrane proteins residing within the plasma membrane are transported via the secretory (exocytotic) pathway. The general transport routes of the secretory pathway are well established. The transport of membrane proteins starts with their integration into the ER membrane. The ribosomes synthesizing membrane proteins are targeted to the ER membrane, and the nascent chains are co-translationally integrated into the bilayer, i.e., they are inserted while their synthesis is in progress. During ER insertion, the orientation (topology) of the proteins in the membrane is determined. Proteins are folded, and their folding state is checked by a quality control system that allows only correctly folded forms to leave the ER. Misfolded or incompletely folded forms are retained, transported back to the cytosol and finally subjected to proteolysis. Correctly folded proteins are transported in the membranes of vesicles through the ER/Golgi intermediate compartment (ERGIC) and the individual compartments of the Golgi apparatus ( cis, medial, trans) to the plasma membrane. In this review, the current knowledge of the first stages of the intracellular trafficking of membrane proteins will be summarized. This "early secretory pathway" includes the processes of ER insertion, topology determination, folding, quality control and the transport to the Golgi apparatus. Mutations in the genes of membrane proteins frequently lead to misfolded forms that are recognized and retained by the quality control system. Such mutations may cause inherited diseases like cystic fibrosis or retinitis pigmentosa. In the second part of this review, the clinical implications of the early secretory pathway will be discussed. Finally, new pharmacological strategies to rescue misfolded and transport-defective membrane proteins will be outlined.

Congenital sucrase-isomaltase deficiency because of an accumulation of the mutant enzyme in the endoplasmic reticulum

BACKGROUND & AIMS

Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive human disorder characterized by reduced activities of the brush border enzyme sucrase-isomaltase (SI). Here, we elucidate the pathogenesis of a new variant of CSID at the cellular and molecular level.

METHODS

Assessment of the CSID phenotype was achieved by enzymatic activity measurements, biosynthetic labeling of intestinal biopsy specimens, immunoprecipitation of SI, and immunoelectronmicroscopy. The putative mutation was identified by sequencing of the SI cDNA isolated by RT-PCR from intestinal biopsy samples. The function of the mutation was verified by immunoprecipitation and confocal microscopy of transiently transfected cells.

RESULTS

Biosynthetic labeling and immunoelectron microscopy reveal a predominant localization of SI in the endoplasmic reticulum (ER) similar to phenotype I of CSID. Unlike phenotype I, however, a partial conversion of SI to a complex glycosylated mature form takes place. The SI cDNA in this phenotype revealed 3 mutations, 2 of which, Val to Phe at residue 15 and Ala to Thr at residue 231, had no effect on the structure or function of SI. By contrast, the third mutation resulted in an exchange of leucine by proline at position 620 (L620P) and revealed in transfected COS cells structural features and subcellular localization similar to the phenotype identified in the patient's enterocytes.

CONCLUSIONS

This is the first identification at the molecular and subcellular levels of a novel variant of CSID in which SI accumulates predominantly in the ER, and a minor proportion is further processed and transported to the apical membrane of enterocytes.

Disaccharidase activities in dyspeptic children: biochemical and molecular investigations of maltase-glucoamylase activity

BACKGROUND

Maltase-glucoamylase enzyme plays an important role in starch digestion. Glucoamylase deficiency is reported to cause chronic diarrhea in infants, but its role in dyspeptic children is unknown.

METHODS

Glucoamylase and other disaccharidase specific activities were assayed from duodenal biopsy specimens in 44 children aged 0.5-18 years (mean, 10 +/- 5 years) undergoing endoscopy to evaluate dyspeptic symptoms. All subjects had normal duodenal histology. Intestinal organ culture was used to evaluate synthesis and processing of maltase-glucoamylase. Sequencing of the maltase-glucoamylase coding region was performed in subjects with low activity or variation of isoform in organ culture.

RESULTS

Twenty-two of the dyspeptic children had one or more disaccharidases with low specific activity. Twelve subjects (28%) had low activity of glucoamylase. Eight subjects had low activities of glucoamylase, sucrase, and lactase. Low glucoamylase activity was not correlated with the isoform phenotype of maltase-glucoamylase as described by metabolic labeling and sodium dodecyl sulfate electrophoresis. Novel nucleotide changes were not detected in one subject with low glucoamylase activity or in two subjects with variant isoforms of maltase-glucoamylase peptides.

CONCLUSION

Twelve of 44 dyspeptic children had low specific activity of duodenal maltase-glucoamylase. Eight of these children had low specific activity of all measured disaccharidases.

Sucrase is an intramolecular chaperone located at the C-terminal end of the sucrase-isomaltase enzyme complex

The sucrase-isomaltase enzyme complex (pro-SI) is a type II integral membrane glycoprotein of the intestinal brush border membrane. Its synthesis commences with the isomaltase (IM) subunit and ends with sucrase (SUC). Both domains reveal striking structural similarities, suggesting a pseudo-dimeric assembly of a correctly folded and an enzymatically active pro-SI. The impact of each domain on the folding and function of pro-SI has been analyzed by individual expression and coexpression of the individual subunits. SUC acquires correct folding, enzymatic activity and transport competence and is secreted into the external milieu independent of the presence of IM. By contrast, IM persists as a mannose-rich polypeptide that interacts with the endoplasmic reticulum resident molecular chaperone calnexin. This interaction is disrupted when SUC is coexpressed with IM, indicating that SUC competes with calnexin for binding of IM. The interaction between SUC and the membrane-anchored IM leads to maturation of IM and blocks the secretion of SUC into the external milieu. We conclude that SUC plays a role as an intramolecular chaperone in the context of the pro-SI protein. To our knowledge all intramolecular chaperones so far identified are located at the N-terminal end. SUC is therefore the first C-terminally located intramolecular chaperone in mammalian cells.

Congenital sucrase-isomaltase deficiency presenting with failure to thrive, hypercalcemia, and nephrocalcinosis

BACKGROUND

Disaccharide Intolerance Type I (Mendelian Interance in Man database: *222900) is a rare inborn error of metabolism resulting from mutation in sucrase-isomaltase (Enzyme Catalyzed 3.2.1.48). Usually, infants with SI deficiency come to attention because of chronic diarrhea and nutritional evidence of malabsorption.

CASE PRESENTATION

We describe an atypical presentation of this disorder in a 10-month-old infant. In addition to chronic diarrhea, the child displayed severe and chronic hypercalcemia, the evaluation of which was negative. An apparently coincidental right orbital hemangioma was detected. Following identification of the SI deficiency, an appropriately sucrose-restricted, but normal calcium diet regimen was instituted which led to cessation of diarrhea, substantial weight gain, and resolution of hypercalcemia.

CONCLUSIONS

This case illustrates that, similar to congenital lactase deficiency (Mendelian Interance in Man database: *223000, Alactasia, Hereditary Disaccharide Intolerance Type II), hypercalcemia may complicate neonatal Sucrase-Isomaltase deficiency. Hypercalcemia in the presence of chronic diarrhea should suggest disaccharide intolerance in young infants.

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.

Role of the membrane-proximal O-glycosylation site in sorting of the human receptor for neurotrophins to the apical membrane of MDCK cells

We have analyzed the respective roles of the stalk and/or the O-glycosylation sites in apical sorting by producing partially deleted mutants in this region of the human receptor for neurotrophins (P75(NTR)). The mere presence of O-glycosylations was not sufficient for efficient delivery to the apical surface since changing the stalk domain of P75(NTR) for the heavily O-glycosylated stalk from human decay-accelerating factor led to random distribution of the chimera. The presence of O-glycosylations, however, was a prerequisite for exit from the ER and protection from intracellular cleavage since a P75(NTR) containing the non O-glycosylated stalk of the human placental alkaline phosphatase was not transported to the cell surface but was cleaved and secreted from the basolateral side. Deletion of the membrane-proximal part of the stalk showed a more dramatic reversal of polarity of P75(NTR) than the deletion of the distal part. Furthermore, moving the first putative O-glycosylation site (T216) two amino acids away from the membrane resulted in a loss of apical polarity of P75(NTR), suggesting that an important clue for apical sorting resides in this part of the stalk. This loss of apical polarity paralleled a loss of association of P75(NTR) mutants with Lubrol rafts. These data indicate that the position of O-glycans in the proximal part of the stalk domain of P75(NTR) is crucial for apical sorting and may regulate association with apical rafts.