Hydrophobic interactions of brush border alkaline phosphatases: the role of phosphatidyl inositol

Hypophosphatasia diagnosis: current state of the art and proposed diagnostic criteria for children and adults

BACKGROUND

This manuscript provides a summary of the current evidence to support the criteria for diagnosing a child or adult with hypophosphatasia (HPP). The diagnosis of HPP is made on the basis of integrating clinical features, laboratory profile, radiographic features of the condition, and DNA analysis identifying the presence of a pathogenic variant of the tissue nonspecific alkaline phosphatase gene (ALPL). Often, the diagnosis of HPP is significantly delayed in both adults and children, and updated diagnostic criteria are required to keep pace with our evolving understanding regarding the relationship between ALPL genotype and associated HPP clinical features.

METHODS

An International Working Group (IWG) on HPP was formed, comprised of a multidisciplinary team of experts from Europe and North America with expertise in the diagnosis and management of patients with HPP. Methodologists (Romina Brignardello-Petersen and Gordon Guyatt) and their team supported the IWG and conducted systematic reviews following the GRADE methodology, and this provided the basis for the recommendations.

RESULTS

The IWG completed systematic reviews of the literature, including case reports and expert opinion papers describing the phenotype of patients with HPP. The published data are largely retrospective and include a relatively small number of patients with this rare condition. It is anticipated that further knowledge will lead to improvement in the quality of genotype-phenotype reporting in this condition.

CONCLUSION

Following consensus meetings, agreement was reached regarding the major and minor criteria that can assist in establishing a clinical diagnosis of HPP in adults and children.

Hypophosphatasia: Biological and Clinical Aspects, Avenues for Therapy

Hypophosphatasia (HPP) is a rare inherited systemic metabolic disease caused by mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. TNSALP is expressed in the liver, kidney and bone, and its substrates include TNSALP inorganic pyrophosphate, pyridoxal-5'-phosphate (PLP)/vitamin B6 and phosphoethanolamine (PEA). Autosomal recessive and dominant forms of the disease result in a range of clinical entities. Major hallmarks are low alkaline phosphatase (ALP) and elevated PLP and PEA levels. Very severe infantile forms of HPP cause premature death as a result of respiratory insufficiency and also present with hypo-mineralisation leading to deformed limbs with, in some cases, the near-absence of bones and skull altogether. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency are indicative of a poor prognosis. Craniosynostosis is frequent. HPP leads to an unusual presentation of rickets with high levels of calcium and phosphorus, resulting in hypercalciuria, nephrocalcinosis and low ALP levels. Hypercalcaemic crisis, failure to thrive and growth retardation are concerns in infants. Fractures are common in both infantile and adult forms of the disease, concomitantly occurring with unexplained chronic pain and fatigue. Dental clinical presentations, which include the premature loss of teeth, are also commonly found in HPP and specifically manifest as odontohypophosphatasia. A novel enzyme therapy for human HPP, asfotase alfa, which is specifically targeted to mineralised tissues, has been developed in the past decades. While this treatment seems very promising, especially for infantile HPP, many questions regarding its long-term effects, the management of treatment, and any potential secondary adverse effects remain unresolved.

Clinical Forms and Animal Models of Hypophosphatasia

Hypophosphatasia (HPP) is due to mutations of the tissue non-specific alkaline phosphatase (TNAP) gene expressed in the liver, kidney, and bone. TNAP substrates include inorganic pyrophosphate cleaved into inorganic phosphate (Pi) in bone, pyridoxal-5'-phosphate (PLP), the circulating form of vitamin B6, and phosphoethanolamine (PEA). As an autosomal recessive or dominant disease, HPP results in a range of clinical forms. Its hallmarks are low alkaline phosphatase (AP) and elevated PLP and PEA levels. Perinatal HPP may cause early death with respiratory insufficiency and hypomineralization resulting in deformed limbs and sometimes near-absence of bones and skull. Infantile HPP is diagnosed before 6 months of life. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency in the brain indicate poor prognosis. Craniosynostosis is frequent. Unlike in other forms of rickets, calcium and phosphorus are not decreased, resulting in hypercalciuria and nephrocalcinosis. Hypercalcemic crisis may occur. Failure to thrive and growth retardation are concerns. In infantile and adult forms of HPP, non-traumatic fractures may be the prominent manifestation, with otherwise unexplained chronic pain. Progressive myopathy has been described. Dental manifestations with early loss of teeth are usual in HPP and in a specific form, odontohypophosphatasia. HPP has been studied in knock-out mice models which mimic its severe form. Animal models have made a major contribution to the development of an original enzyme therapy for human infantile HPP, which is however essentially targeted at mineralized tissues. Better knowledge of its extraskeletal manifestations, including pain and neurological symptoms, is therefore required.

Novel ALPL genetic alteration associated with an odontohypophosphatasia phenotype

Hypophosphatasia (HPP) is an inherited disorder of mineral metabolism caused by mutations in ALPL, encoding tissue non-specific alkaline phosphatase (TNAP). Here, we report the molecular findings from monozygotic twins, clinically diagnosed with tooth-specific odontohypophosphatasia (odonto-HPP). Sequencing of ALPL identified two genetic alterations in the probands, including a heterozygous missense mutation c.454C>T, leading to change of arginine 152 to cysteine (p.R152C), and a novel heterozygous gene deletion c.1318_1320delAAC, leading to the loss of an asparagine residue at codon 440 (p.N440del). Clinical identification of low serum TNAP activity, dental abnormalities, and pedigree data strongly suggests a genotype-phenotype correlation between p.N440del and odonto-HPP in this family. Computational analysis of the p.N440del protein structure revealed an alteration in the tertiary structure affecting the collagen-binding site (loop 422-452), which could potentially impair the mineralization process. Nevertheless, the probands (compound heterozygous: p.[N440del];[R152C]) feature early-onset and severe odonto-HPP phenotype, whereas the father (p.[N440del];[=]) has only moderate symptoms, suggesting p.R152C may contribute or predispose to a more severe dental phenotype in combination with the deletion. These results assist in defining the genotype-phenotype associations for odonto-HPP, and further identify the collagen-binding site as a region of potential structural importance for TNAP function in the biomineralization.

Functional characterization of a novel mutation localized in the start codon of the tissue-nonspecific alkaline phosphatase gene

Hypophosphatasia (HPP) is a rare inborn disease caused by different mutations in the tissue-nonspecific alkaline phosphatase (ALPL) gene. Previous studies showed that gene mutations could exhibit a dominant negative effect leading to a mild HPP phenotype in heterozygous carriers. In the present report we describe the clinical and functional studies of a novel mutation localized in the start codon of transcript variant 1 of the ALPL gene from a female adult heterozygous carrier. The mutation results in translation of an N-terminally truncated protein, which might be identical to the deduced protein from ALPL transcript variant 2. When overexpressed in HEK-293 cells it does not exhibit any enzymatic activity and has no significant effect on the wild type ALPL protein. Furthermore it is not attached to the cell membrane. Due to the loss of the signal peptide an intracellular misrouting and a premature degradation is obvious. Hence the new isoform deposited in the database does not produce an active protein as it is the case in the natural mutation of our patient. Since the mutation does not produce a dominant negative protein in heterozygous carriers, the clinical phenotype in our patient and her relatives is very mild with only unspecific myalgia. However the patient developed bone marrow edema of both femoral heads during lactation after delivery of a healthy child, indicating a risk to develop alterations of bone metabolism in challenge situations. Her sister complains of identical symptoms, her father shows distinct symptoms of odonto-hypophosphatasia. The question if or if not carriers of ALPL mutations in general or only with distinct genotypes can be symptomatic in normal life or in challenge situations requires systematic clinical studies.

Prolonged survival and phenotypic correction of Akp2(-/-) hypophosphatasia mice by lentiviral gene therapy

Hypophosphatasia (HPP) is an inherited systemic skeletal disease caused by mutations in the gene encoding the tissue-nonspecific alkaline phosphatase (TNALP) isozyme. The clinical severity of HPP varies widely, with symptoms including rickets and osteomalacia. TNALP knockout (Akp2(-/-)) mice phenotypically mimic the severe infantile form of HPP; that is, TNALP-deficient mice are born with a normal appearance but die by 20 days of age owing to growth failure, hypomineralization, and epileptic seizures. In this study, a lentiviral vector expressing a bone-targeted form of TNALP was injected into the jugular vein of newborn Akp2(-/-) mice. We found that alkaline phosphatase activity in the plasma of treated Akp2(-/-) mice increased and remained at high levels throughout the life of the animals. The treated Akp2(-/-) mice survived for more than 10 months and demonstrated normal physical activity and a healthy appearance. Epileptic seizures were completely inhibited in the treated Akp2(-/-) mice, and X-ray examination of the skeleton showed that mineralization was significantly improved by the gene therapy. These results show that severe infantile HPP in TNALP knockout mice can be treated with a single injection of lentiviral vector during the neonatal period.

Modification of ceftibuten transport by the addition of non-ionic surfactants

The effects of non-ionic surfactants on the carrier-mediated transport of ceftibuten by rat intestinal brush-border membrane vesicles (BBMVs) were investigated. Ceftibuten uptake by BBMVs was measured by a rapid filtration technique. The concentration of surfactants for the uptake experiments was determined by a decrease in the turbidity of BBMV suspension and by the release of an impermeable probe, 2',7'-bis(carboxyethyl)-4(5)-carboxyfluorescein, from the vesicle inside. In fact, the surfactant concentration of 0. 03% (w/v) was selected to maintain the stability of BBMVs. The extent of ceftibuten uptake by BBMVs with various surfactants was correlated with their physicochemical properties, i.e. hydrophile-lipophile balance (HLB), critical micelle concentration (c.m.c.), average diameter of micelle colloid, and polydispersity determined by particle size distribution. The surfactants used were divided into two groups on the basis of polydispersity index (d(w)/d(n)), i.e. low polydispersity (d(w)/d(n) congruent with1) and high polydispersity d(w)/d(n)2). The ceftibuten uptake due to the addition of surfactants with low polydispersity increased with a decrease in the HLB number. These results indicate that the ceftibuten transport is modulated by the size distribution and hydrophobicity of surfactants. In addition, the effects of surfactants on the membrane lipid fluidity monitored by diphenylhexatriene (DPH) and trimethylammonium diphenylhexatriene (TMA-DPH) were investigated. There was significant correlation between ceftibuten uptake and the fluorescence anisotropy of TMA-DPH-labeled membranes due to the addition of surfactants with low polydispersity (r=-0.81, P<0.0001). These results suggest that surfactants with low polydispersity, in part, increase or decrease the outer membrane leaflet, thereby enhancing or suppressing the ceftibuten transport by BBMVs, and that ceftibuten transport caused by surfactants with low polydispersity may be strongly dependent on the hydrophobic interaction.

Conjugated bile salts regulate turnover of rat intestinal brush border membrane hydrolases

The mechanisms whereby the conjugated bile salts regulate the activities of the brush border membrane hydrolases and its physiological significance were investigated in rat small intestine, and comparisons were made with the action of pancreatic protease. Rat brush border membrane proteins were metabolically labeled with [35S]methionine, and isolated brush border membrane was incubated with taurocholate or pancreatic elastase. The activity of solubilized hydrolases was assayed and the molecular forms of the hydrolases were examined by SDS-PAGE. The activity and protein bands of alkaline phosphatase and sucrase-isomaltase were solubilized by taurocholate, while alkaline phosphatase was not solubilized by elastase. Solubilized sucrase-isomaltase molecules were proteolytically degraded by elastase, whereas the intact molecule of sucrase-isomaltase was solubilized by taurocholate. Next the physiological role of bile salts in brush border membrane hydrolase turnover were investigated using metabolic labeling of brush border membrane hydrolase and immunoprecipitation in biliary diversion rats. After three days of biliary diversion, a significant increase in alkaline phosphatase activity was observed. Although synthesis of alkaline phosphatase in biliary diversion rats was similar to that observed in control rats, biliary diversion rats showed 1.5-fold slower turnover of alkaline phosphatase when compared with control rats. These results suggest that conjugated bile salts in the intestinal lumen may cause a rapid turnover of brush border membrane hydrolases, which may be increased by the enhanced enzyme degradation. The mechanisms for the enhanced degradation appeared to be solubilization of hydrolases caused by the detergent activity of bile salts. Therefore, conjugated bile salts may play an important physiological role in the regulation of expression of the protease-resistant enzymes such as alkaline phosphatase.

Inhibition of in vitro mineralization in osteoblastic cells and in mouse tooth germ by phosphatidylinositol-specific phospholipase C

We have examined the effect of phosphatidylinositol-specific phospholipase C (PIPLC) on the in vitro mineralization during cultivation of both osteoblast-like MC3T3-E1 cells and the lower molar tooth germ from mouse embryo. PIPLC not only caused the release of alkaline phosphatase (ALP) from cell membranes into the culture medium but also produced a disturbance of in vitro mineralization in MC3T3-E1 cells and in mouse embryonic tooth germ. These findings strongly suggest that ALP, functioning as an ectoenzyme, is involved in the physiological mineralization process of both bone and teeth.

Developmental and dietary induction of the 90K subunit of rat intestinal phytase

The activities of phytase and alkaline phosphatase in the intestine gradually increased in parallel during development of rats, but the 70K and 90K subunits were expressed differentially; only the 70K subunit was detected at birth, whereas the 90K subunit appeared at the weaning period (3 weeks after birth). When rats were forced to wean at 18 days old and fed laboratory chow, the enzyme activity increased markedly and the 90K subunit appeared within 1 day. These findings suggest that weaning is involved in the change in the subunit composition. Increases in the enzyme activity and amount of the 90K subunit were significantly delayed by feeding weanling animals on casein diet, but induced significantly by feeding them on casein diet supplemented with phytate. Thus induction of the 90K subunit seems to be accelerated by intake of phytic acid in the diet. The Km value of the enzyme from suckling rats for phytate was 5.25 mM, while that of adult rats was 0.213 mM. In contrast, the Km value for p-nitrophenyl phosphate (PNPP) was constant during development. The phytase activity of suckling rats did not show a distinct pH-dependence. These findings suggest that the 90K subunit may play some important roles in expressing an efficient phytase activity.

Evidence for glycosylphosphatidylinositol anchoring of intralumenal alkaline phosphatase of the calf intestine

1. Considerable amounts of intestinal alkaline phosphatase (AP) were found intralumenally in all animal species investigated, i.e. calf, pig, goat, rat, mouse, guinea pig, hen and carp. The ratios between the total activity of AP found intralumenally and the total intestinal activity vary considerably. Calves and pigs show the highest, i.e. 0.77 and 0.44, respectively, while rodents have much lower ratios. Only 20-34% of the intralumenal alkaline phosphatase (IAP) of the calf and pig is soluble and not within the sediment after centrifugation at 135,000 x g for 60 min. whereas the IAP of rodents is soluble in the range of 60-72% of the total IAP. 2. For the IAP of the mucosa and chyme of calf, all criteria were found which are generally used, indicating a glycosylphosphatidylinositol (GlcPtdIns) anchor as proved by strong hydrophobicity using Triton X-114 phase partitioning, phenyl-Sepharose binding and enzyme aggregation, and the susceptibility to phosphatidylinositol-specific phospholipase C (PtdIns-PLC) and papain digestion. 3. More than 80% of the mucosa alkaline phosphatase (MAP) of the proximal part of the intestine and of the particulate fraction of IAP exhibit these criteria indicating the presence of the GlcPtdIns-anchor structure, whereas the anchor content of the soluble intralumenal enzyme decreases from the pylorus to the ileocecal junction. 4. MAP partially purified to a specific activity of 1747 IU/mg retains the anchor structure. 5. The results presented indicate that the release of large amounts of AP into the chyme is realized without splitting the GlcPtdIns anchor. The possible intralumenal function of this form of AP is discussed.

Selective inhibition of duodenal and jejunal villous cell alkaline phosphatase by the duodenal ulcerogen cysteamine

We have found that cysteamine-HCl, a potent duodenal ulcerogen, after a single subcutaneous injection (30 mg/100 g body weight), inhibited villous cell duodenal and jejunal alkaline phosphatase (APase) under in vivo and under in vitro conditions. The duodenal and jejunal crypt-cell APase was not susceptible to cysteamine inhibition. Ileal APase from both the villous and the crypt cells was unaffected by cysteamine. Tissue-nonspecific APase from the kidney and liver was not affected by cysteamine either. The differences in tissue and cellular accumulation of cysteamine, the submolecular differences in APase molecules, and its anatomical localization in mucosal cells along the small intestine could explain the different degrees of susceptibility to cysteamine inhibition. The extent of duodenal APase inhibition by cysteamine was highly pH-dependent and varied from 5% to 85% within a pH range of 7.5-10.5. A shift in pH optimum from 9.6 to 9.3 was found in the presence of cysteamine. The inhibition of duodenal villous cell APase was greatly dependent on cysteamine concentration (Ki = 2.65 mM). At a fixed concentration of cysteamine it was not influenced by substrate concentration. Cysteamine did not change the Km value for duodenal APase but did decrease its Vmax to 46% and 15% of the controls when added in the assay or injected subcutaneously, respectively, indicating that the inhibition was of the linear, 'noncompetitive' type. Somehow cysteamine increased the requirement in the activation energy for substrate hydrolysis as well. The data indicate that macromolecular transformations could take place in the mucosal cells of duodenum after cysteamine administration.

Stimulation of canine kidney BBMV ATPase activity by acidic pH in the presence of Zn2+: an ATPase activity distinct from transport ATPases and alkaline phosphatase that may be an ecto-ATPase

Renal brush border membrane vesicles (BBMV) of the dog possess at least two ATPase activities. In the present study, we have examined the effect of pH, ions, and inhibitors on the activity of ATPase in BBMV. Two different sets of conditions were identified that produced stimulation of ATPase activity. A unique stimulation of BBMV ATPase activity occurred at acidic pH in the presence of 1 mM ZnCl2. In the absence of Zn2+, a second ATPase activity was stimulated by alkaline pH values with peak stimulation occurring between pH 8.5 and 9.0. The results suggest that the alkaline pH-stimulated hydrolysis of ATP probably represents the activity of BBMV alkaline phosphatase. The unique acidic pH + Zn2(+)-stimulated ATPase activity must represent the activity of a second protein other than the alkaline phosphatase, since purified alkaline phosphatase did not show this activity. The biochemical identity and physiological function of this renal BBMV ATPase activity remain to be determined, but it may be an ecto-ATPase.

Translocation of intestinal alkaline phosphatase in streptozotocin-induced diabetic rats

1. We determined the organ of origin and possible mechanism of translocation into the circulation of alkaline phosphatase (ALPase) in the diabetic rat. 2. Experimental diabetes was induced by injection of streptozotocin, resulting in a 8.2-fold elevation in serum ALPase activity. In this case, the major ALPase isozyme detected in serum was intestinal ALPase. 3. In in vitro experimental systems, ALPase was readily released from the duodenal plasma membrane by bacterial phosphatidylinositol-specific-phospholipase C (PI-PLase C) but little if any was released from the ileal membrane. 4. Serum and ileal ALPases were identical in terms of molecular size, whereas duodenal ALPase clearly differed from the serum enzyme. 5. Based on an investigation of the sugar moiety, more of the fraction having higher concanavalin A affinity was found in serum ALPase than with in the case of either of the intestinal ALPases. Serum and intestinal ALPases also differed slightly regarding isoelectric points. 6. Consequently, these data suggest that the serum ALPase of the diabetic rat is derived from ileal ALPase, and it is unlikely that the appearance of ALPase in the circulation is simply the result of solubilization by the action of PI-PLase C or phospholipase D.

Release of carcinoembryonic antigen from human colon cancer cells by phosphatidylinositol-specific phospholipase C

Carcinoembryonic antigen (CEA) is released from colon cancer cells into the circulation where it is monitored clinically as an indicator of the recurrence or progression of cancer. We have studied the mechanism of CEA membrane attachment and release using the human colonic adenocarcinoma cell line LS-174T, specimens of human colon cancers, and serum from colon cancer patients. CEA release by cells in vitro and in vivo is associated with the conversion of CEA from a membrane-bound, hydrophobic molecule to a soluble, hydrophilic form with no apparent decrease in molecular mass. When LS-174T cell membranes were incubated with various buffers, proteases, and phospholipases, the only agents that released CEA and converted it to the hydrophilic form were preparations of phosphatidylinositol-specific phospholipase C (PI-PLC). Both [3H]ethanolamine and [3H]palmitate could be incorporated metabolically into CEA but only palmitate was released by treatment with PI-PLC, consistent with the presence of a glycosyl-phosphatidylinositol linkage. PI-PLC treatment also release significant quantities of CEA from living monolayers and from seven human colon cancer specimens. These experiments suggest that cellular CEA is anchored to membranes by a covalent linkage to a membrane phosphatidylinositol molecule, and that an endogenous phospholipase may be important for releasing CEA in vitro and in vivo.