Differential effects of the lipidic and ionic microenvironment on NPP1's phosphohydrolase and phosphodiesterase activities

Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) is an enzyme present in matrix vesicles (MV). NPP1 participates on the regulation of bone formation by producing pyrophosphate (PPi) from adenosine triphosphate (ATP). Here, we have used liposomes bearing dipalmitoylphosphatidylcholine (DPPC), sphingomyelin (SM), and cholesterol (Chol) harboring NPP1 to mimic the composition of MV lipid rafts to investigate ionic and lipidic influence on NPP1 activity and mineral propagation. Atomic force microscopy (AFM) revealed that DPPC-liposomes had spherical and smooth surface. The presence of SM and Chol elicited rough and smooth surface, respectively. NPP1 insertion produced protrusions in all the liposome surface. Maximum phosphodiesterase activity emerged at 0.082 M ionic strength, whereas maximum phosphomonohydrolase activity arose at low ionic strength. Phosphoserine-Calcium Phosphate Complex (PS-CPLX) and amorphous calcium-phosphate (ACP) induced mineral propagation in DPPC- and DPPC:SM-liposomes and in DPPC:Chol-liposomes, respectively. Mineral characterization revealed the presence of bands assigned to HAp in the mineral propagated by NPP1 harbored in DPPC-liposomes without nucleators or in DPPC:Chol-liposomes with ACP nucleators. These data show that studying how the ionic and lipidic environment affects NPP1 properties is important, especially for HAp obtained under controlled conditions in vitro.

Delivery of Alkaline Phosphatase Promotes Periodontal Regeneration in Mice

Factors regulating the ratio of pyrophosphate (PP_i) to phosphate (P_i) modulate biomineralization. Tissue-nonspecific alkaline phosphatase (TNAP) is a key promineralization enzyme that hydrolyzes the potent mineralization inhibitor PP_i. The goal of this study was to determine whether TNAP could promote periodontal regeneration in bone sialoprotein knockout mice (Ibsp^-/- mice), which are known to have a periodontal disease phenotype. Delivery of TNAP was accomplished either systemically (through a lentiviral construct expressing a mineral-targeted TNAP-D₁₀ protein) or locally (through addition of recombinant human TNAP to a fenestration defect model). Systemic TNAP-D₁₀ delivered by intramuscular injection at 5 d postnatal (dpn) increased circulating alkaline phosphatase (ALP) levels in Ibsp^-/- mice by 5-fold at 30 dpn, with levels returning to normal by 60 dpn when tissues were evaluated by micro-computed tomography and histology. Local delivery of recombinant human TNAP to fenestration defects in 5-wk-old wild type (WT) and Ibsp^-/- mice did not alter long-term circulating ALP levels, and tissues were evaluated by micro-computed tomography and histology at postoperative day 45. Systemic and local delivery of TNAP significantly increased alveolar bone volume (20% and 37%, respectively) and cementum thickness (3- and 42-fold) in Ibsp^-/- mice, with evidence for periodontal ligament attachment and bone/cementum marker localization. Local delivery significantly increased regenerated cementum and bone in WT mice. Addition of 100-μg/mL bovine intestinal ALP to culture media to increase ALP in vitro increased media P_i concentration, mineralization, and Spp1 and Dmp1 marker gene expression in WT and Ibsp^-/- OCCM.30 cementoblasts. Use of phosphonoformic acid, a nonspecific inhibitor of sodium P_i cotransport, indicated that effects of bovine intestinal ALP on mineralization and marker gene expression were in part through P_i transport. These findings show for the first time through multiple in vivo and in vitro approaches that pharmacologic modulation of P_i/PP_i metabolism can overcome periodontal breakdown and accomplish regeneration.

Dental defects in the primary dentition associated with hypophosphatasia from biallelic ALPL mutations

ALPL encodes tissue-nonspecific alkaline phosphatase (TNAP), an enzyme expressed in bone, teeth, liver, and kidney. ALPL loss-of-function mutations cause hypophosphatasia (HPP), an inborn error-of-metabolism that produces skeletal and dental mineralization defects. Case reports describe widely varying dental phenotypes, making it unclear how HPP comparatively affects the three unique dental mineralized tissues: enamel, dentin, and cementum. We hypothesized that HPP affected all dental mineralized tissues and aimed to establish quantitative measurements of dental tissues in a subject with HPP. The female proband was diagnosed with HPP during childhood based on reduced alkaline phosphatase activity (ALP), mild rachitic skeletal effects, and premature primary tooth loss. The diagnosis was subsequently confirmed genetically by the presence of compound heterozygous ALPL mutations (exon 5: c.346G>A, p.A116T; exon 10: c.1077C>G, p.I359M). Dental defects in 8 prematurely exfoliated primary teeth were analyzed by high resolution micro-computed tomography (micro-CT) and histology. Similarities to the Alpl^-/- mouse model of HPP were identified by additional analyses of murine dentoalveolar tissues. Primary teeth from the proband exhibited substantial remaining root structure compared to healthy control teeth. Enamel and dentin densities were not adversely affected in HPP vs. control teeth. However, analysis of discrete dentin regions revealed an approximate 10% reduction in the density of outer mantle dentin of HPP vs. control teeth. All 4 incisors and the molar lacked acellular cementum by micro-CT and histology, but surprisingly, 2 of 3 prematurely exfoliated canines exhibited apparently normal acellular cementum. Based on dentin findings in the proband's teeth, we examined dentoalveolar tissues in a mouse model of HPP, revealing that the delayed initiation of mineralization in the incisor mantle dentin was associated with a broader lack of circumpulpal dentin mineralization. This study describes a quantitative approach to measure effects of HPP on dental tissues. This approach has uncovered a previously unrecognized novel mantle dentin defect in HPP, as well as a surprising and variable cementum phenotype within the teeth from the same HPP subject.

Lipid composition modulates ATP hydrolysis and calcium phosphate mineral propagation by TNAP-harboring proteoliposomes

Bone biomineralization is mediated by a special class of extracellular vesicles, named matrix vesicles (MVs), released by osteogenic cells. The MV membrane is enriched in sphingomyelin (SM), cholesterol (Chol) and tissue non-specific alkaline phosphatase (TNAP) compared with the parent cells' plasma membrane. TNAP is an ATP phosphohydrolase bound to cell and MV membranes via a glycosylphosphatidylinositol (GPI) anchor. Previous studies have shown that the lipid microenvironment influences the catalytic activity of enzymes incorporated into lipid bilayers. However, there is a lack of information about how the lipid microenvironment controls the ability of MV membrane-bound enzymes to induce mineral precipitation. Herein, we used TNAP-harboring proteoliposomes made of either pure dimyristoylphosphatidylcholine (DMPC) or DMPC mixed with either Chol, SM or both of them as MV biomimetic systems to evaluate how the composition modulates the lipid microenvironment and, in turn, TNAP incorporation into the lipid bilayer by means of calorimetry. These results were correlated with the proteoliposomes' catalytic activity and ability to induce the precipitation of amorphous calcium phosphate (ACP) in vitro. DMPC:SM proteoliposomes displayed the highest efficiency of mineral propagation, apparent affinity for ATP and substrate hydrolysis efficiency, which correlated with their highest degree of membrane organization (highest ΔH), among the tested proteoliposomes. Results obtained from turbidimetry and Fourier transformed infrared (FTIR) spectroscopy showed that the tested proteoliposomes induced ACP precipitation with the order DMPC:SM>DMPC:Chol:SM≈DMPC:Chol>DMPC which correlated with the lipid organization and the presence of SM in the proteoliposome membrane. Our study arises important insights regarding the physical properties and role of lipid organization in MV-mediated mineralization.

Genetic and pharmacologic modulation of cementogenesis via pyrophosphate regulators

Pyrophosphate (PP_i) serves as a potent and physiologically important regulator of mineralization, with systemic and local concentrations determined by several key regulators, including: tissue-nonspecific alkaline phosphatase (ALPL gene; TNAP protein), the progressive ankylosis protein (ANKH; ANK), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1; ENPP1). Results to date have indicated important roles for PP_i in cementum formation, and we addressed several gaps in knowledge by employing genetically edited mouse models where PP_i metabolism was disrupted and pharmacologically modulating PP_i in a PP_i-deficient mouse model. We demonstrate that acellular cementum growth is inversely proportional to PP_i levels, with reduced cementum in Alpl KO (increased PP_i levels) mice and excess cementum in Ank KO mice (decreased PP_i levels). Moreover, simultaneous ablation of Alpl and Ank results in reestablishment of functional cementum in dKO mice. Additional reduction of PP_i by dual deletion of Ank and Enpp1 does not further increase cementogenesis, and PDL space is maintained in part through bone modeling/remodeling by osteoclasts. Our results provide insights into cementum formation and expand our knowledge of how PP_i regulates cementum. We also demonstrate for the first time that pharmacologic manipulation of PP_i through an ENPP1-Fc fusion protein can regulate cementum growth, supporting therapeutic interventions targeting PP_i metabolism.

Hypophosphatasia (adult form): quantitation of serum alkaline phosphatase isoenzyme activity in a large kindred

We used heat inactivation, L-phenylalanine inhibition, and electrophoresis on polyacrylamide gel and cellulose acetate membranes--with and without use of specific antisera against the liver-bone, intestinal, and placental isoenzymes--to distinguish and quantitate the different alkaline phosphatase isoenzymes in sera from 23 adult members of a kindred affected by the adult form of hypophosphatasia. Nine subjects had values for total activity more than two standard deviations below the mean values for age- and sex-matched normal persons. Bone isoenzyme was diminished in all nine, whereas liver isoenzyme was subnormal in only four. Phosphoethanolamine and phosphoserine in the urine of eight hypophosphatasemic individuals correlated inversely with both total and liver alkaline phosphatase activity in their serum, but not with the activity of the bone isoenzyme. Total activity in the serum of adult kindred members correlated best with the circulating liver isoenzyme activity. The findings suggest that altered hepatic metabolism is responsible for the increased urinary excretion of phosphoethanolamine, and perhaps phosphoserine, in hypophosphatasia.

Increased calcium uptake and improved trabecular bone properties in intestinal alkaline phosphatase knockout mice

Previous studies have demonstrated a negative correlation between intestinal alkaline phosphatase (IAP) activity and calcium (Ca) absorption in the gut, as IAP acts as a protective mechanism inhibiting high Ca entry into enterocytes, preventing Ca overload. Here we evaluated Ca absorption and bone properties in knockout mice (KO) completely devoid of duodenal IAP (Akp3 ^-/- mice). Female C57BL/6 control mice (WT, n = 7) and KO mice (n = 10) were used to determine Ca absorption in vivo and by in situ isolated duodenal loops followed by histomorphometric analysis of duodenal villi and crypts. Bone mineral density, morphometry, histomorphometry and trabecular connectivity and biomechanical properties were measured on bones. We observed mild atrophy of the villi with lower absorption surface and a significantly higher Ca uptake in KO mice. While no changes were seen in cortical bone, we found better trabecular connectivity and biomechanical properties in the femurs of KO mice compared to WT mice. Our data indicate that IAP KO mice display higher intestinal Ca uptake, which over time appears to correlate with a positive effect on the biomechanical properties of trabecular bone.

Inhibition of arterial medial calcification and bone mineralization by extracellular nucleotides: The same functional effect mediated by different cellular mechanisms

Arterial medial calcification (AMC) is thought to share some outward similarities to skeletal mineralization and has been associated with the transdifferentiation of vascular smooth muscle cells (VSMCs) to an osteoblast-like phenotype. ATP and UTP have previously been shown to inhibit bone mineralization. This investigation compared the effects of extracellular nucleotides on calcification in VSMCs with those seen in osteoblasts. ATP, UTP and the ubiquitous mineralization inhibitor, pyrophosphate (PP_i ), dose dependently inhibited VSMC calcification by ≤85%. Culture of VSMCs in calcifying conditions was associated with an increase in apoptosis; treatment with ATP, UTP, and PP_i reduced apoptosis to levels seen in non-calcifying cells. Extracellular nucleotides had no effect on osteoblast viability. Basal alkaline phosphatase (TNAP) activity was over 100-fold higher in osteoblasts than VSMCs. ATP and UTP reduced osteoblast TNAP activity (≤50%) but stimulated VSMC TNAP activity (≤88%). The effects of extracellular nucleotides on VSMC calcification, cell viability and TNAP activity were unchanged by deletion or inhibition of the P2Y₂ receptor. Conversely, the actions of ATP/UTP on bone mineralization and TNAP activity were attenuated in osteoblasts lacking the P2Y₂ receptor. Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) hydrolyses ATP and UTP to produce PP_i . In both VSMCs and osteoblasts, deletion of NPP1 blunted the inhibitory effects of extracellular nucleotides suggesting involvement of P2 receptor independent pathways. Our results show that although the overall functional effect of extracellular nucleotides on AMC and bone mineralization is similar there are clear differences in the cellular mechanisms mediating these actions.

Osteopontin regulates dentin and alveolar bone development and mineralization

The periodontal complex is essential for tooth attachment and function and includes the mineralized tissues, cementum and alveolar bone, separated by the unmineralized periodontal ligament (PDL). To gain insights into factors regulating cementum-PDL and bone-PDL borders and protecting against ectopic calcification within the PDL, we employed a proteomic approach to analyze PDL tissue from progressive ankylosis knock-out (Ank^-/-) mice, featuring reduced PP_i, rapid cementogenesis, and excessive acellular cementum. Using this approach, we identified the matrix protein osteopontin (Spp1/OPN) as an elevated factor of interest in Ank^-/- mouse molar PDL. We studied the role of OPN in dental and periodontal development and function. During tooth development in wild-type (WT) mice, Spp1 mRNA was transiently expressed by cementoblasts and strongly by alveolar bone osteoblasts. Developmental analysis from 14 to 240days postnatal (dpn) indicated normal histological structures in Spp1^-/- comparable to WT control mice. Microcomputed tomography (micro-CT) analysis at 30 and 90dpn revealed significantly increased volumes and tissue mineral densities of Spp1^-/- mouse dentin and alveolar bone, while pulp and PDL volumes were decreased and tissue densities were increased. However, acellular cementum growth was unaltered in Spp1^-/- mice. Quantitative PCR of periodontal-derived mRNA failed to identify potential local compensators influencing cementum in Spp1^-/- vs. WT mice at 26dpn. We genetically deleted Spp1 on the Ank^-/- mouse background to determine whether increased Spp1/OPN was regulating periodontal tissues when the PDL space is challenged by hypercementosis in Ank^-/- mice. Ank^-/-; Spp1^-/- double deficient mice did not exhibit greater hypercementosis than that in Ank^-/- mice. Based on these data, we conclude that OPN has a non-redundant role regulating formation and mineralization of dentin and bone, influences tissue properties of PDL and pulp, but does not control acellular cementum apposition. These findings may inform therapies targeted at controlling soft tissue calcification.

Overlapping functions of bone sialoprotein and pyrophosphate regulators in directing cementogenesis

Although acellular cementum is essential for tooth attachment, factors directing its development and regeneration remain poorly understood. Inorganic pyrophosphate (PP_i), a mineralization inhibitor, is a key regulator of cementum formation: tissue-nonspecific alkaline phosphatase (Alpl/TNAP) null mice (increased PP_i) feature deficient cementum, while progressive ankylosis protein (Ank/ANK) null mice (decreased PP_i) feature increased cementum. Bone sialoprotein (Bsp/BSP) and osteopontin (Spp1/OPN) are multifunctional extracellular matrix components of cementum proposed to have direct and indirect effects on cell activities and mineralization. Studies on dentoalveolar development of Bsp knockout (Bsp^-/-) mice revealed severely reduced acellular cementum, however underlying mechanisms remain unclear. The similarity in defective cementum phenotypes between Bsp^-/- mice and Alpl^-/- mice (the latter featuring elevated PP_i and OPN), prompted us to examine whether BSP is operating by modulating PP_i-associated genes. Genetic ablation of Bsp caused a 2-fold increase in circulating PP_i, altered mRNA expression of Alpl, Spp1, and Ank, and increased OPN protein in the periodontia. Generation of a Bsp knock-out (KO) cementoblast cell line revealed significantly decreased mineralization capacity, 50% increased PP_i in culture media, and increased Spp1 and Ank mRNA expression. While addition of 2μg/ml recombinant BSP altered Spp1, Ank, and Enpp1 expression in cementoblasts, changes resulting from this dose were not dependent on the integrin-binding RGD motif or MAPK/ERK signaling pathway. Decreasing PP_i by genetic ablation of Ank on the Bsp^-/- mouse background reestablished cementum formation, allowing >3-fold increased acellular cementum volume compared to wild-type (WT). However, deleting Ank did not fully compensate for the absence of BSP. Bsp^-/-; Ank^-/- double-deficient mice exhibited mean 20-27% reduced cementum thickness and volume compared to Ank^-/- mice. From these data, we conclude that the perturbations in PP_i metabolism are not solely driving the cementum pathology in Bsp^-/- mice, and that PP_i is more potent than BSP as a cementum regulator, as shown by the ability to override loss of BSP by lowering PP_i. We propose that BSP and PP_i work in concert to direct mineralization in cementum and likely other mineralized tissues.

Conditional Alpl Ablation Phenocopies Dental Defects of Hypophosphatasia

Loss-of-function mutations in ALPL result in hypophosphatasia (HPP), an inborn error of metabolism that causes defective skeletal and dental mineralization. ALPL encodes tissue-nonspecific alkaline phosphatase, an enzyme expressed in bone, teeth, liver, and kidney that hydrolyzes the mineralization inhibitor inorganic pyrophosphate. As Alpl-null mice die before weaning, we aimed to generate mouse models of late-onset HPP with extended life spans by engineering a floxed Alpl allele, allowing for conditional gene ablation (conditional knockout [cKO]) when crossed with Cre recombinase transgenic mice. The authors hypothesized that targeted deletion of Alpl in osteoblasts and selected dental cells ( Col1a1-cKO) or deletion in chondrocytes, osteoblasts, and craniofacial mesenchyme ( Prx1-cKO) would phenocopy skeletal and dental manifestations of late-onset HPP. Col1a1-cKO and Prx1-cKO mice were viable and fertile, and they did not manifest the epileptic seizures characteristic of the Alpl^-/- model of severe infantile HPP. Both cKO models featured normal postnatal body weight but significant reduction as compared with wild type mice by 8 to 12 wk. Plasma alkaline phosphatase for both cKO models at 24 wk was reduced by approximately 75% as compared with controls. Radiography revealed profound skeletal defects in cKO mice, including rachitic changes, hypomineralized long bones, deformations, and signs of fractures. Microcomputed tomography confirmed quantitative differences in cortical and trabecular bone, including decreased cortical thickness and mineral density. Col1a1-cKO mice exhibited classic signs of HPP dentoalveolar disease, including short molar roots with thin dentin, lack of acellular cementum, and osteoid accumulation in alveolar bone. Prx1-cKO mice exhibited the same array of periodontal defects but featured less affected molar dentin. Both cKO models exhibited reduced alveolar bone height and 4-fold increased numbers of osteoclast-like cells versus wild type at 24 wk, consistent with HPP-associated periodontal disease. These novel models of late-onset HPP can inform on long-term skeletal and dental manifestations and will provide essential tools to further studies of etiopathologies and therapeutic interventions.

Effects of GPI-anchored TNAP on the dynamic structure of model membranes

Tissue-nonspecific alkaline phosphatase (TNAP) plays a crucial role during skeletal mineralization, and TNAP deficiency leads to the soft bone disease hypophosphatasia. TNAP is anchored to the external surface of the plasma membranes by means of a GPI (glycosylphosphatidylinositol) anchor. Membrane-anchored and solubilized TNAP displays different kinetic properties against physiological substrates, indicating that membrane anchoring influences the enzyme function. Here, we used Electron Spin Resonance (ESR) measurements along with spin labeled phospholipids to probe the possible dynamic changes prompted by the interaction of GPI-anchored TNAP with model membranes. The goal was to systematically analyze the ESR data in terms of line shape changes and of alterations in parameters such as rotational diffusion rates and order parameters obtained from non-linear least-squares simulations of the ESR spectra of probes incorporated into DPPC liposomes and proteoliposomes. Overall, the presence of TNAP increased the dynamics and decreased the ordering in the three distinct regions probed by the spin labeled lipids DOPTC (headgroup), and 5- and 16-PCSL (acyl chains). The largest change was observed for 16-PCSL, thus suggesting that GPI-anchored TNAP can give rise to long reaching modifications that could influence membrane processes halfway through the bilayer.

Role of PHOSPHO1 in Periodontal Development and Function

The tooth root and periodontal apparatus, including the acellular and cellular cementum, periodontal ligament (PDL), and alveolar bone, are critical for tooth function. Cementum and bone mineralization is regulated by factors including enzymes and extracellular matrix proteins that promote or inhibit hydroxyapatite crystal growth. Orphan Phosphatase 1 (Phospho1, PHOSPHO1) is a phosphatase expressed by chondrocytes, osteoblasts, and odontoblasts that functions in skeletal and dentin mineralization by initiating deposition of hydroxyapatite inside membrane-limited matrix vesicles. The role of PHOSPHO1 in periodontal formation remains unknown and we aimed to determine its functional importance in these tissues. We hypothesized that the enzyme would regulate proper mineralization of the periodontal apparatus. Spatiotemporal expression of PHOSPHO1 was mapped during periodontal development, and Phospho1(-/-) mice were analyzed using histology, immunohistochemistry, in situ hybridization, radiography, and micro-computed tomography. The Phospho1 gene and PHOSPHO1 protein were expressed by active alveolar bone osteoblasts and cementoblasts during cellular cementum formation. In Phospho1(-/-) mice, acellular cementum formation and mineralization were unaffected, whereas cellular cementum deposition increased although it displayed delayed mineralization and cementoid. Phospho1(-/-) mice featured disturbances in alveolar bone mineralization, shown by accumulation of unmineralized osteoid matrix and interglobular patterns of protein deposition. Parallel to other skeletal sites, deposition of mineral-regulating protein osteopontin (OPN) was increased in alveolar bone in Phospho1(-/-) mice. In contrast to the skeleton, genetic ablation of Spp1, the gene encoding OPN, did not ameliorate dentoalveolar defects in Phospho1(-/-) mice. Despite alveolar bone mineralization defects, periodontal attachment and function appeared undisturbed in Phospho1(-/-) mice, with normal PDL architecture and no evidence of bone loss over time. This study highlights the role of PHOSPHO1 in mineralization of alveolar bone and cellular cementum, further revealing that acellular cementum formation is not substantially regulated by PHOSPHO1 and likely does not rely on matrix vesicle-mediated initiation of mineralization.

Periodontal Defects in the A116T Knock-in Murine Model of Odontohypophosphatasia

Mutations in ALPL result in hypophosphatasia (HPP), a disease causing defective skeletal mineralization. ALPL encodes tissue nonspecific alkaline phosphatase (ALP), an enzyme that promotes mineralization by reducing inorganic pyrophosphate, a mineralization inhibitor. In addition to skeletal defects, HPP causes dental defects, and a mild clinical form of HPP, odontohypophosphatasia, features only a dental phenotype. The Alpl knockout (Alpl (-/-)) mouse phenocopies severe infantile HPP, including profound skeletal and dental defects. However, the severity of disease in Alpl (-/-) mice prevents analysis at advanced ages, including studies to target rescue of dental tissues. We aimed to generate a knock-in mouse model of odontohypophosphatasia with a primarily dental phenotype, based on a mutation (c.346G>A) identified in a human kindred with autosomal dominant odontohypophosphatasia. Biochemical, skeletal, and dental analyses were performed on the resulting Alpl(+/A116T) mice to validate this model. Alpl(+/A116T) mice featured 50% reduction in plasma ALP activity compared with wild-type controls. No differences in litter size, survival, or body weight were observed in Alpl(+/A116T) versus wild-type mice. The postcranial skeleton of Alpl(+/A116T) mice was normal by radiography, with no differences in femur length, cortical/trabecular structure or mineral density, or mechanical properties. Parietal bone trabecular compartment was mildly altered. Alpl(+/A116T) mice featured alterations in the alveolar bone, including radiolucencies and resorptive lesions, osteoid accumulation on the alveolar bone crest, and significant differences in several bone properties measured by micro-computed tomography. Nonsignificant changes in acellular cementum did not appear to affect periodontal attachment or function, although circulating ALP activity was correlated significantly with incisor cementum thickness. The Alpl(+/A116T) mouse is the first model of odontohypophosphatasia, providing insights on dentoalveolar development and function under reduced ALP, bringing attention to direct effects of HPP on alveolar bone, and offering a new model for testing potential dental-targeted therapies in future studies.

Compounded PHOSPHO1/ALPL deficiencies reduce dentin mineralization

Phosphatases are involved in bone and tooth mineralization, but their mechanisms of action are not completely understood. Tissue-nonspecific alkaline phosphatase (TNAP, ALPL) regulates inhibitory extracellular pyrophosphate through its pyrophosphatase activity to control mineral propagation in the matrix; mice without TNAP lack acellular cementum, and have mineralization defects in dentin, enamel, and bone. PHOSPHO1 is a phosphatase found within membrane-bounded matrix vesicles in mineralized tissues, and double ablation of Alpl and Phospho1 in mice leads to a complete absence of skeletal mineralization. Here, we describe mineralization abnormalities in the teeth of Phospho1(-/-) mice, and in compound knockout mice lacking Phospho1 and one allele of Alpl (Phospho1(-/-);Alpl(+/-) ). In wild-type mice, PHOSPHO1 and TNAP co-localized to odontoblasts at early stages of dentinogenesis, coincident with the early mineralization of mantle dentin. In Phospho1 knockout mice, radiography, micro-computed tomography, histology, and transmission electron microscopy all demonstrated mineralization abnormalities of incisor dentin, with the most remarkable findings being reduced overall mineralization coincident with decreased matrix vesicle mineralization in the Phospho1(-/-) mice, and the almost complete absence of matrix vesicles in the Phospho1(-/-);Alpl(+/-) mice, whose incisors showed a further reduction in mineralization. Results from this study support prominent non-redundant roles for both PHOSPHO1 and TNAP in dentin mineralization.

Proteoliposomes in nanobiotechnology

Proteoliposomes are systems that mimic lipid membranes (liposomes) to which a protein has been incorporated or inserted. During the last decade, these systems have gained prominence as tools for biophysical studies on lipid-protein interactions as well as for their biotechnological applications. Proteoliposomes have a major advantage when compared with natural membrane systems, since they can be obtained with a smaller number of lipidic (and protein) components, facilitating the design and interpretation of certain experiments. However, they have the disadvantage of requiring methodological standardization for incorporation of each specific protein, and the need to verify that the reconstitution procedure has yielded the correct orientation of the protein in the proteoliposome system with recovery of its functional activity. In this review, we chose two proteins under study in our laboratory to exemplify the steps necessary for the standardization of the reconstitution of membrane proteins in liposome systems: (1) alkaline phosphatase, a protein with a glycosylphosphatidylinositol anchor, and (2) Na,K-ATPase, an integral membrane protein. In these examples, we focus on the production of the specific proteoliposomes, as well as on their biochemical and biophysical characterization, with emphasis on studies of lipid-protein interactions. We conclude the chapter by highlighting current prospects of this technology for biotechnological applications, including the construction of nanosensors and of a multi-protein nanovesicular biomimetic to study the processes of initiation of skeletal mineralization.

Thermodynamic properties and characterization of proteoliposomes rich in microdomains carrying alkaline phosphatase

Tissue-nonspecific alkaline phosphatase (TNAP) is associated to the plasma membrane via a GPI-anchor and plays a key role in the biomineralization process. In plasma membranes, most GPI-anchored proteins are associated with "lipid rafts", ordered microdomains enriched in sphingolipids, glycosphingolipids and cholesterol. In order to better understand the role of lipids present in rafts and their interactions with GPI-anchored proteins, the insertion of TNAP into different lipid raft models was studied using dipalmitoylphosphatidylcholine (DPPC), cholesterol (Chol), sphingomyelin (SM) and ganglioside (GM1). Thus, the membrane models studied were binary systems (9:1 molar ratio) containing DPPC:Chol, DPPC:SM and DPPC:GM1, ternary systems (8:1:1 molar ratio) containing DPPC:Chol:SM, DPPC:Chol:GM1 and DPPC:SM:GM1 and finally, a quaternary system (7:1:1:1 molar ratio) containing DPPC:Chol:SM:GM1. Calorimetry analysis of the liposomes and proteoliposomes indicate that lateral phase segregation could be noted only in the presence of cholesterol, with the formation of cholesterol-rich microdomains centered above Tc=41.5°C. The presence of GM1 and SM into DPPC-liposomes influenced mainly ΔH and Δt(1/2) values. The gradual increase in the complexity of the systems decreased the activity of the enzyme incorporated. The presence of the enzyme also fluidifies the systems, as seen by the intense reduction in ∆H values, but do not alter Tc values significantly. Therefore, the study of different microdomains and its biophysical characterization may contribute to the knowledge of the interactions between the lipids present in MVs and its interactions with TNAP.

Enzyme replacement therapy prevents dental defects in a model of hypophosphatasia

Hypophosphatasia (HPP) occurs from loss-of-function mutation in the tissue-non-specific alkaline phosphatase (TNALP) gene, resulting in extracellular pyrophosphate accumulation that inhibits skeletal and dental mineralization. TNALP-null mice (Akp2(-/-)) phenocopy human infantile hypophosphatasia; they develop rickets at 1 week of age, and die before being weaned, having severe skeletal and dental hypomineralization and episodes of apnea and vitamin B(6)-responsive seizures. Delay and defects in dentin mineralization, together with a deficiency in acellular cementum, are characteristic. We report the prevention of these dental abnormalities in Akp2(-/-) mice receiving treatment from birth with daily injections of a mineral-targeting, human TNALP (sALP-FcD(10)). sALP-FcD(10) prevented hypomineralization of alveolar bone, dentin, and cementum as assessed by micro-computed tomography and histology. Osteopontin--a marker of acellular cementum--was immuno-localized along root surfaces, confirming that acellular cementum, typically missing or reduced in Akp2(-/-) mice, formed normally. Our findings provide insight concerning how acellular cementum is formed on tooth surfaces to effect periodontal ligament attachment to retain teeth in their osseous alveolar sockets. Furthermore, they provide evidence that this enzyme-replacement therapy, applied early in post-natal life--where the majority of tooth root development occurs, including acellular cementum formation--could prevent the accelerated tooth loss seen in individuals with HPP.

Proteoliposomes as matrix vesicles' biomimetics to study the initiation of skeletal mineralization

During the process of endochondral bone formation, chondrocytes and osteoblasts mineralize their extracellular matrix by promoting the formation of hydroxyapatite (HA) seed crystals in the sheltered interior of membrane-limited matrix vesicles (MVs). Ion transporters control the availability of phosphate and calcium needed for HA deposition. The lipidic microenvironment in which MV-associated enzymes and transporters function plays a crucial physiological role and must be taken into account when attempting to elucidate their interplay during the initiation of biomineralization. In this short mini-review, we discuss the potential use of proteoliposome systems as chondrocyte- and osteoblast-derived MVs biomimetics, as a means of reconstituting a phospholipid microenvironment in a manner that recapitulates the native functional MV microenvironment. Such a system can be used to elucidate the interplay of MV enzymes during catalysis of biomineralization substrates and in modulating in vitro calcification. As such, the enzymatic defects associated with disease-causing mutations in MV enzymes could be studied in an artificial vesicular environment that better mimics their in vivo biological milieu. These artificial systems could also be used for the screening of small molecule compounds able to modulate the activity of MV enzymes for potential therapeutic uses. Such a nanovesicular system could also prove useful for the repair/treatment of craniofacial and other skeletal defects and to facilitate the mineralization of titanium-based tooth implants.

Intestinal alkaline phosphatase preserves the normal homeostasis of gut microbiota

BACKGROUND AND AIMS

The intestinal microbiota plays a critical role in maintaining human health; however, the mechanisms governing the normal homeostatic number and composition of these microbes are largely unknown. Previously it was shown that intestinal alkaline phosphatase (IAP), a small intestinal brush border enzyme, functions as a gut mucosal defence factor limiting the translocation of gut bacteria to mesenteric lymph nodes. In this study the role of IAP in the preservation of the normal homeostasis of the gut microbiota was investigated.

METHODS

Bacterial culture was performed in aerobic and anaerobic conditions to quantify the number of bacteria in the stools of wild-type (WT) and IAP knockout (IAP-KO) C57BL/6 mice. Terminal restriction fragment length polymorphism, phylogenetic analyses and quantitative real-time PCR of subphylum-specific bacterial 16S rRNA genes were used to determine the compositional profiles of microbiotas. Oral supplementation of calf IAP (cIAP) was used to determine its effects on the recovery of commensal gut microbiota after antibiotic treatment and also on the colonisation of pathogenic bacteria.

RESULTS

IAP-KO mice had dramatically fewer and also different types of aerobic and anaerobic microbes in their stools compared with WT mice. Oral supplementation of IAP favoured the growth of commensal bacteria, enhanced restoration of gut microbiota lost due to antibiotic treatment and inhibited the growth of a pathogenic bacterium (Salmonella typhimurium).

CONCLUSIONS

IAP is involved in the maintenance of normal gut microbial homeostasis and may have therapeutic potential against dysbiosis and pathogenic infections.

Contribution of matrix vesicles and alkaline phosphatase to ectopic bone formation

Endochondral calcification involves the participation of matrix vesicles (MVs), but it remains unclear whether calcification ectopically induced by implants of demineralized bone matrix also proceeds via MVs. Ectopic bone formation was induced by implanting rat demineralized diaphyseal bone matrix into the dorsal subcutaneous tissue of Wistar rats and was examined histologically and biochemically. Budding of MVs from chondrocytes was observed to serve as nucleation sites for mineralization during induced ectopic osteogenesis, presenting a diameter with Gaussian distribution with a median of 306 +/- 103 nm. While the role of tissue-nonspecific alkaline phosphatase (TNAP) during mineralization involves hydrolysis of inorganic pyrophosphate (PPi), it is unclear how the microenvironment of MV may affect the ability of TNAP to hydrolyze the variety of substrates present at sites of mineralization. We show that the implants contain high levels of TNAP capable of hydrolyzing p-nitrophenylphosphate (pNPP), ATP and PPi. The catalytic properties of glycosyl phosphatidylinositol-anchored, polidocanol-solubilized and phosphatidylinositol-specific phospholipase C-released TNAP were compared using pNPP, ATP and PPi as substrates. While the enzymatic efficiency (k cat/Km) remained comparable between polidocanol-solubilized and membrane-bound TNAP for all three substrates, the k cat/Km for the phosphatidylinositol-specific phospholipase C-solubilized enzyme increased approximately 108-, 56-, and 556-fold for pNPP, ATP and PPi, respectively, compared to the membrane-bound enzyme. Our data are consistent with the involvement of MVs during ectopic calcification and also suggest that the location of TNAP on the membrane of MVs may play a role in determining substrate selectivity in this micro-compartment.

Conserved epitopes in human and mouse tissue-nonspecific alkaline phosphatase. Second report of the ISOBM TD-9 workshop

A panel of 19 monoclonal antibodies (MAbs) against human tissue-nonspecific (liver/bone/kidney) alkaline phosphatase (TNAP) was obtained through the ISOBM TD-9 workshop. In the present study, the reactivity of these MAbs has been characterized against mouse TNAP. A mouse embryonic stem cell line, frozen sections of long bones, alkaline phosphatase extracted from mouse bone, and serum were used as the source of TNAP for individual assays. Each MAb was tested for immunoreactivity to mouse TNAP by Western blot analysis, immunohistochemistry and enzyme immunoassay. Antibodies 314 and 315 reacted strongly with mouse TNAP in Western blots, while all other antibodies were negative. By immunohistochemistry, antibodies 314, 315 and 333 produced strong positive staining using frozen sections, while antibody 334 was moderately positive. Enzyme immunoassays indicated that MAb 333 was also able to bind to serum TNAP. These antibodies represent very useful reagents to study the pathophysiological expression of TNAP in mouse tissues and in mouse serum.

Monoclonal antibodies against tissue-nonspecific alkaline phosphatase. Report of the ISOBM TD9 workshop

Nineteen monoclonal antibodies (MAbs) against tissue-nonspecific (liver/bone/kidney) alkaline phosphatase (TNALP) were investigated in the ISOBM TD-9 Workshop. These MAbs were generated with antigens obtained from human bone tissue (n = 9), human osteosarcoma cell lines (SaOS-2 and TPX; n = 7) and human liver tissue (n = 3). The evaluation included the following antigen forms: (a) commercially available preparations of human bone ALP (BALP) and liver ALP (LALP); (b) human BALP isoforms, B/I, B1 and B2; and (c) soluble secreted epitope-tagged recombinant human TNALP (setTNALP) expressed in COS-1, osteosarcoma (SaOS-2) and hepatoma (Huh2) cell lines. In addition, 16 TNALP mutant cDNAs corresponding to a wide spectrum of reported hypophosphatasia mutations were used in an attempt to map specific immunoreactive epitopes on the surface of the TNALP molecule. The TD-9 MAbs were evaluated by immunoradiometric (IRMA) assays, cross-inhibition and different enzyme immunoassay designs. No indications of explicit tissue discriminatory immunoreactivities of the investigated MAbs against TNALP were found. However, certain IRMA combinations of MAbs increased the specificity of BALP measurements. All MAbs bound to the three BALP isoforms B/I, B1 and B2, but none of the investigated MAbs were specific for any of the isoforms. Significant differences were, however, found in immunoreactivity between these isoforms, with cross-reactivities ranging from 21 to 109% between the two major BALP isoforms B1 and B2. Desialylation with neuraminidase significantly increased the MAb affinity for the BALP isoforms B/I, B1 and B2, and also decreased the observed differences in cross-reactivity between these isoforms. We suggest, therefore, that the MAb affinity is dependent on the amount/number of terminal sialic acid residues located at the five putative N-glycosylation sites. Based on the overall results, we present a putative three-dimensional model of the TNALP molecule with positioning of the four major antigenic domains (designated A-D) of the investigated MAbs. The TNALP molecule is depicted as a homodimer, hence most, but not necessarily all, epitopes are displayed twice. The antigenic domains were positioned with the following assumptions: domain A was positioned close to the active site since most of these MAbs interfered with the catalytic activity. Interestingly, both MAbs included in the commercial BALP kits were grouped with domain A. Moreover, 4 of the 5 putative N-glycosylation sites (with terminal sialic acid residues) are located within, or with close proximity to, domain A. Domain B was localized at the top flexible loop (crown domain) of the TNALP molecule. Domain C was clearly defined by the IRMA assay combinations and by site-directed mutants of TNALP to be close to residue E281, which is located near the fourth metal binding site, likely to be occupied by a calcium ion. Domain D was positioned close to residues A115, A162 and E174, but this domain was also close to the GPI anchor site. In conclusion, none of the 19 investigated TD-9 MAbs were entirely specific for BALP or LALP, thus indicating that all MAbs bind mainly to epitopes on the common protein core of BALP and LALP and/or common glycosylated epitopes. However, some MAbs (either single or in combination with other MAbs) work sufficiently well to measure BALP when the assayed samples do not contain elevated levels of LALP.

MUSEAP, a novel reporter gene for the study of long-term gene expression in immunocompetent mice

The improvement of gene therapy vectors would benefit from the availability of a reporter gene that can be used for long-term studies in immunocompetent laboratory animals. We describe the construction and characterization of a novel reporter gene, murine secreted embryonic alkaline phosphatase (MUSEAP). We demonstrate by gene transfer in skeletal muscle of immunocompetent mice that MUSEAP is efficiently secreted and detected in the bloodstream and that injection of an increasing dose of DNA leads to a dose-dependent increase of plasma MUSEAP activity. We also show that the expression of MUSEAP under the control of a constitutive promoter is stable for 1 year and that the activity of MUSEAP in the bloodstream reflects the changes in the transcription rate of its gene. These properties make MUSEAP the only reporter gene that can be used for somatic gene transfer into immunocompetent mice in order to study the impact of gene transfer vectors of metabolic, developmental or environmental factors on long-term gene expression.

Abnormal vitamin B6 metabolism in alkaline phosphatase knock-out mice causes multiple abnormalities, but not the impaired bone mineralization

The tissue non-specific alkaline phosphatase (TNAP) knock-out mouse is a model of infantile hypophosphatasia displaying impaired bone mineralization, epileptic seizures, apnoea, abnormal apoptosis in the thymus, abnormal lumbar nerve roots, and postnatal death. Administration of vitamin B6 suppresses the epileptic seizures in TNAP-/- mice. This paper examines to what extent the diverse abnormalities seen in these mice are due to impaired utilization of vitamin B6, using two complementary approaches: administration of vitamin B6 to TNAP null mice and deprivation of vitamin B6 in wild-type and TNAP heterozygous mice. Administration of exogenous pyridoxal HCl delayed the onset of epileptic attacks and increased the life span of TNAP-/- mice. The episodes of apnoea ceased and the appearance of lumbar nerve roots improved, but hypomineralization and accumulation of osteoid continued to worsen with age. Control mice fed a vitamin B6-depleted diet developed epileptic seizures indistinguishable from those observed in TNAP-/- mice, abnormal apoptosis in the thymus, and thinning of the nerve roots, but showed no evidence of bone mineralization abnormalities. Depletion of vitamin B6 did not affect the ability of primary cultures of osteoblasts to deposit bone mineral in vitro. While abnormal metabolism of vitamin B6 explains many of the abnormalities in this mouse model of infantile hypophosphatasia, it is not the basis of the abnormal mineralization that characterizes this disease.

Functional characterization of osteoblasts and osteoclasts from alkaline phosphatase knockout mice

Tissue nonspecific alkaline phosphatase (TNAP) knockout (ko) mice manifest defects in bone mineralization that mimic the phenotypic abnormalities of infantile hypophosphatasia. In this article, we have searched for phenotypic differences between calvarial osteoblasts and osteoclasts in wild-type (wt), heterozygous and homozygous TNAP null mice. In vitro release of 45Ca from calvarial bones, with and without stimulation with parathyroid hormone (PTH), revealed no functional difference between osteoclasts from the three TNAP genotypes. Studies of primary cultures of TNAP+/+, TNAP+/-, and TNAP-/- calvarial osteoblasts revealed no differences in the rate of protein synthesis or in the expression levels of messenger RNAs (mRNAs) for osteopontin (OP), osteocalcin (OC), collagen type I, core binding factor alpha1 (Cbfa 1), N-cadherin, Smad 5, and Smad 7. Release of interleukin-6 (IL-6) from calvarial osteoblasts under basal conditions and after stimulation with PTH, tumor necrosis factor alpha (TNF-alpha) or IL-1beta was similar in all genotypes. The amount of cyclic adenosine monophosphate (cAMP) accumulation also was comparable. However, although cultures of primary TNAP-/- osteoblasts were able to form cellular nodules as well as TNAP positive osteoblasts do, they lacked the ability to mineralize these nodules in vitro. Mineralization also was delayed in TNAP+/- osteoblast cultures compared with cultures of wt osteoblasts. Incubation with media supplemented with recombinant TNAP, but not with enzymatically inactive TNAP, restored mineralization in ko osteoblast cultures. Our data provide evidence that osteoblasts in TNAP null mice differentiate normally but are unable to initiate mineralization in vitro. The fact that even heterozygous osteoblasts show delayed mineralization provides a rationale for the presence of bone disease in carriers of hypophosphatasia.

Effects of disruption of the embryonic alkaline phosphatase gene on preimplantation development of the mouse

Embryonic alkaline phosphatase (EAP) is expressed during the preimplantation period of mouse development; however, its function is unknown. To determine whether the absence of an EAP gene affects development of preimplantation embryos, we studied mice homozygous for the disrupted EAP gene (EAP.ko mice). Time to reach morphologically definedpreimplantation stages, preimplantation loss, cell count, gestation length, and litter size were monitored, and it was found that EAP.ko embryos have slower development and higher rates of degeneration during in vitro preimplantation development. In vivo, EAP.ko mice had a longergestation, smaller litter size, and fewer cells at 93 hr after human chorionic gonadotropin injection. Furthermore, there was no compensation for the absence of EAP gene in EAP.ko embryos by other isozymes of alkaline phosphatase. We conclude that the presence of an active EAP gene is beneficial for preimplantation development of the mouse embryo, and its absence leads to fewer blastocysts in vitro, delayed parturition, and reduced litter size in vivo.

A novel frameshift mutation (2436insT) produces an immediate stop codon in the autosomal dominant polycystic kidney disease 2 (PKD2) gene

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disorder that can be caused by mutations in at least three different genes. Several mutations have been identified in PKD1 and PKD2 genes. Most of the mutations found in PKD2 gene are predicted to cause premature termination of the protein.

METHODS

We analysed an Argentinian family characterized previously as PKD2. The PKD2 gene was amplified from genomic DNA using 17 primer pairs and the products were analysed by heteroduplex analysis. PCR products that showed a variation by heteroduplex analysis were sequenced directly. The mutation was confirmed by sequencing relatives. The segregation of the mutation in this family was verified by restriction endonuclease digestion of PCR products obtained from genomic DNA of all family members. Results and conclusions. Here, we report a novel mutation present in an Argentinian family characterized as PKD2 by linkage analysis. The mutation, shared by all affected members of the family, is a thymidine insertion at position 2436 of the gene, which results in a translation frameshift and creates an immediate stop codon. This mutation is expected to lead to a truncated protein that lacks the interacting domain with the PKD1 gene product. The thymidine insertion abolished a Ddel restriction site, allowing a rapid test for detection of PKD2 carriers in the family.

Alkaline phosphatase knock-out mice recapitulate the metabolic and skeletal defects of infantile hypophosphatasia

Hypophosphatasia is an inborn error of metabolism characterized by deficient activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP) and skeletal disease due to impaired mineralization of cartilage and bone matrix. We investigated two independently generated TNSALP gene knock-out mouse strains as potential models for hypophosphatasia. Homozygous mice (-/-) had < 1% of wild-type plasma TNSALP activity; heterozygotes had the predicted mean of approximately 50%. Phosphoethanolamine, inorganic pyrophosphate, and pyridoxal 5'-phosphate are putative natural substrates for TNSALP and all were increased endogenously in the knock-out mice. Skeletal disease first appeared radiographically at approximately 10 days of age and featured worsening rachitic changes, osteopenia, and fracture. Histologic studies revealed developmental arrest of chondrocyte differentiation in epiphyses and in growth plates with diminished or absent hypertrophic zones. Progressive osteoidosis from defective skeletal matrix mineralization was noted but not associated with features of secondary hyperparathyroidism. Plasma and urine calcium and phosphate levels were unremarkable. Our findings demonstrate that TNSALP knock-out mice are a good model for the infantile form of hypophosphatasia and provide compelling evidence for an important role for TNSALP in postnatal development and mineralization of the murine skeleton.

Bispecific antibody-mediated lysis of primary cultures of ovarian carcinoma cells using multiple target antigens

We have shown previously that a bispecific antibody (BsAb) directed against both germ-cell alkaline phosphatase (GCAP) and the CD3 complex on mouse T cells could effectively eliminate GCAP-positive tumor cells in vivo using an immunocompetent mouse model. However, some GCAP-negative tumor cells were still able to grow, suggesting that BsAb therapy, when used in a clinical setting, could benefit from targeting several tumor markers to prevent outgrowth of tumor cells lacking a targeted marker. To test this hypothesis, we developed an in vitro model based on primary human ovarian carcinoma (OC) cultures and BsAbs directed against human T cells and several tumor markers [placental alkaline phosphatase (PLAP), GCAP, folate-binding protein (FBP) and CA19.9]. OC cells, isolated from primary tumors, were co-cultured with human peripheral blood mononuclear cells in the presence or absence of various concentrations of BsAbs against PLAP/GCAP, FBP and CA19.9 administered separately or in combination. Results derived from 18 primary OC samples showed that the combination treatment was better than or equally effective as the best single BsAB treatment in 60% of cases. Sometimes targeting FBP, PLAP/GCAP or CA19.9 alone was superior to targeting all simultaneously. Combining each BsAb with a low dose of IL-2 was always beneficial. These results indicate that before using a specific BsAb in the clinic, it is important to determine the optimal BsAb for each patient using this in vitro assay on cells from the removed tumor mass.

Heterogeneity in alkaline phosphatase isozyme expression in human testicular germ cell tumours: An enzyme-/immunohistochemical and molecular analysis

In humans, alkaline phosphatases are encoded by one tissue-non-specific alkaline phosphatase (TNAP) gene and three tissue-specific alkaline phosphatase genes, intestinal, placental (PLAP), and germ cell-specific alkaline phosphatase (GCAP). Although the presence of alkaline phosphatases in testicular germ cell tumours (TGCTs) of adolescents and adults has been utilized for both detection and patient monitoring, it is not known in detail which isozymes are expressed. Since alkaline phosphatase is detected in carcinoma in situ (CIS), the common precursor of all TGCTs, it might provide a marker for the early diagnosis of TGCTs. Testicular cancers of germ cell and non-germ cell origin along with testicular parenchyma with and without CIS have been analysed for the expression of the different alkaline phosphatase isozymes. Antibodies to TNAP and PLAP/GCAP showed positivity in CIS, seminoma, and embryonal carcinoma. The heterogeneous staining pattern detected in frozen tissue sections was similar to the pattern found in formalin-fixed, paraffin-embedded material, indicating a biological phenomenon and not a handling artefact. Since PLAP and GCAP cannot be distinguished using immunohistochemistry, the expression of these isozymes was studied at the molecular level using a reverse transcriptase-polymerase chain reaction (RT-PCR) approach, in combination with a primer extension assay. The results show that CIS and seminoma predominantly express GCAP, while in embryonal carcinoma the expression of GCAP versus PLAP varies. Due to the presence of alkaline phosphatase transcripts in normal testicular parenchyma, an RT-PCR-based analysis of alkaline phosphatase is not informative for the early detection of TGCTs in biopsy samples.

Genetic complexity, structure, and characterization of highly active bovine intestinal alkaline phosphatases

Mammalian alkaline phosphatases (APs) display 10-100-fold higher kcat values than do bacterial APs. To begin uncovering the critical residues that determine the catalytic efficiency of mammalian APs, we have compared the sequence of two bovine intestinal APs, i.e. a moderately active isozyme (bovine intestinal alkaline phosphatase, bIAP I, approximately 3,000 units/mg) previously cloned in our laboratory, and a highly active isozyme (bIAP II, approximately 8, 000 units/mg) of hitherto unknown sequence. An unprecedented level of complexity was revealed for the bovine AP family of genes during our attempts to clone the bIAP II cDNA from cow intestinal RNAs. We cloned and characterized two novel full-length IAP cDNAs (bIAP III and bIAP IV) and obtained partial sequences for three other IAP cDNAs (bIAP V, VI, and VII). Moreover, we identified and partially cloned a gene coding for a second tissue nonspecific AP (TNAP-2). However, the cDNA for bIAP II, appeared unclonable. The sequence of the entire bIAP II isozyme was determined instead by a classical protein sequencing strategy using trypsin, carboxypeptidase, and endoproteinase Lys-C, Asp-N, and Glu-C digestions, as well as cyanogen bromide cleavage and NH2-terminal sequencing. A chimeric bIAP II cDNA was then constructed by ligating wild-type and mutagenized fragments of bIAP I, III, and IV to build a cDNA encoding the identified bIAP II sequence. Expression and enzymatic characterization of the recombinant bIAP I, II, III, and IV isozymes revealed average kcat values of 1800, 5900, 4200, and 6100 s-1, respectively. Comparison of the bIAP I and bIAP II sequences identified 24 amino acid positions as likely candidates to explain differences in kcat. Site-directed mutagenesis and kinetic studies revealed that a G322D mutation in bIAP II reduced its kcat to 1300 s-1, while the converse mutation, i.e. D322G, in bIAP I increased its kcat to 5800 s-1. Other mutations in bIAP II had no effect on its kinetic properties. Our data clearly indicate that residue 322 is the major determinant of the high catalytic turnover in bovine IAPs. This residue is not directly involved in the mechanism of catalysis but is spatially sufficiently close to the active site to influence substrate positioning and hydrolysis of the phosphoenzyme complex.

Decreased expression of cold-inducible RNA-binding protein (CIRP) in male germ cells at elevated temperature

Physiological scrotal hypothermia is necessary for normal spermatogenesis and fertility in mammals. Cirp is a recently identified cold-inducible RNA-binding protein that is inducible at 32 degrees C in mouse somatic cells in vitro. Cirp is constitutively expressed in the testis of mouse and structurally highly similar to RBM1, a candidate for the human azoospermia factor. To elucidate the role played by Cirp in spermatogenesis, we investigated its expression levels during spermatogenesis and after heat stress. In the mouse testis, cirp mRNA was detected in the germ cells, and the level varied depending on the stage of differentiation. Also, a high level of Cirp protein was detected immunohistochemically in the nucleus of primary spermatocytes. Expression of Cirp was decreased in the GC-2spd(ts) mouse germ cell line when culture temperature was raised from 32 degrees C to 37 degrees C. When mouse testis was exposed to heat stress by experimental cryptorchidism or immersion of the lower abdomen in warm (42 degrees C) water, the expression of Cirp was decreased in the testis within 6 hours after either treatment. In human testis with varicocele analyzed immunohistochemically, germ cells expressed less Cirp protein than those in the testis without varicocele. These results demonstrated that CIRP expression is down-regulated at elevated temperature in male germ cells of mice and humans. Analysis of Cirp expression in the testes will help elucidate the molecular mechanisms leading to male infertility.

Mammalian alkaline phosphatases are allosteric enzymes

Mammalian alkaline phosphatases (APs) are zinc-containing metalloenzymes encoded by a multigene family and functional as dimeric molecules. Using human placental AP (PLAP) as a paradigm, we have investigated whether the monomers in a given PLAP dimer are subject to cooperativity during catalysis following an allosteric model or act via a half-of-sites model, in which at any time only one single monomer is operative. Wild type and mutant PLAP homodimers and heterodimers were produced by stably transfecting Chinese hamster ovary cells with mutagenized PLAP cDNAs followed by enzyme extraction, purification, and characterization. [Gly429]PLAP manifested negative cooperativity when partially metalated as a consequence of the reduced affinity of the incompletely metalated AP monomers for the substrate. Upon full metalation with Zn2+, however, the negative cooperativity disappeared. To distinguish between an allosteric and a half-of-sites model, a [Gly429]PLAP-[Ser84]PLAP heterodimer was produced by combining monomers displaying high and low sensitivity to the uncompetitive inhibitor L-Leu as well as a [Gly429]PLAP-[Ala92]PLAP heterodimer combining a catalytically active and inactive monomer, respectively. The L-Leu inhibition profile of the [Gly429]PLAP-[Ser84]PLAP heterodimer was intermediate to that for each homodimer as predicted by the allosteric model. Likewise, the [Gly429]PLAP-[Ala92]PLAP heterodimer was catalytically active, confirming that AP monomers act independently of each other. Although heterodimers are structurally asymmetrical, they migrate in starch gels with a smaller than expected weighted electrophoretic mobility, are more stable to heat denaturation than expected, and are more sensitive to L-Leu inhibition than predicted by a strict noncooperative model. We conclude that fully metalated mammalian APs are noncooperative allosteric enzymes but that the stability and catalytic properties of each monomer are controlled by the conformation of the second AP subunit.

Inactivation of two mouse alkaline phosphatase genes and establishment of a model of infantile hypophosphatasia

We report the inactivation, via homologous recombination, of two of the three active mouse alkaline phosphatase genes, i.e., embryonic (EAP) and tissue nonspecific (TNAP). Whereas expression of the EAP isozyme was abolished in all tissues that express EAP developmentally (such as the preimplantation embryo, thymus, and testis), the EAP knock-out mice show no obvious phenotypic abnormalities. They reproduce normally and give birth to live offspring, indicating the nonessential role of EAP during embryonic development. Mice deficient in the TNAP gene mimic a severe form of hypophosphatasia. These TNAP-/- mice are growth impaired, develop epileptic seizures and apnea, and die before weaning. Examination of the tissues indicates abnormal bone mineralization and morphological changes in the osteoblasts, aberrant development of the lumbar nerve roots, disturbances in intestinal physiology, increased apoptosis in the thymus, and abnormal spleens. Our results indicate that, in the mouse, TNAP appears not to be essential for the initial events leading to bone mineral deposition but that TNAP seems to play a role in the maintenance of this process after birth. The other phenotypic manifestations may be a consequence of the lack of TNAP in the developing neural tube between stages E8.5 and E13.5 of embryogenesis. We hypothesize that the autonomic nervous system is compromised in these TNAP-/- mice.

Comparative analysis of cell surface antigens expressed by cell lines derived from human germ cell tumours

The pattern of cell surface antigen expression of a set of cell lines derived from human germ cell tumours and corresponding to various cell phenotypes found within these tumours was studied using immunofluorescence. Twenty-two different antibodies were used. Many of these antibodies have been noted to recognise epitopes that are either preferentially expressed by embryonal carcinoma (EC) cells, or by more differentiated cell types. Using scatter plots and rank correlations, 6 groups of antibodies were distinguished with respect to their staining patterns on the cell lines tested. Several antibodies showed a specific staining pattern in relation to the differentiation state of the cells. Two groups of antibodies included those recognising high m.w. glycoproteins (antibodies TRA-1-60, TRA-1-81, GCTM2, 3-177, K4 and K21) and the ganglioseries glycolipid antigens SSEA-3 and -4 (antibodies MC631 and MC813-70). These antibodies mostly stained EC cells but not other cell types, confirming previously published data. However, one of these groups, comprising antibodies K4 and MC631, was more exclusively associated with the EC cell phenotype than was the other group. Antibodies recognising the liver isozyme of alkaline phosphatase (TRA-2-49 and TRA-2-54) also reacted strongly with most EC cell lines, although they reacted significantly with a number of other cell lines as well, whereas antibodies to the placental isozyme tended to react only weakly with EC cells. The antibodies recognising the ganglioseries glycolipids GD2 and GD3 (VIN2PB22 and VINIS56) preferentially stained cells with neuroectodermal characteristics. Other antibodies showed a heterogeneous staining pattern for the cell lines with different phenotypes. The data obtained from the cell lines were, in general, similar to data obtained from immunohistochemical studies on tissue sections of primary germ cell tumours of the adult testis, including carcinoma in situ.

Bispecific antibody-mediated lysis of placental and germ cell alkaline phosphatase targeted solid tumors in immunocompetent mice

Recently, an immunocompetent in vivo mouse model was developed based on germ cell alkaline phosphatase (GCAP) transgenic (FVB/N x C3H) mice in which both placental alkaline phosphatase (PLAP)+ and GCAP+ solid MO4 tumors develop. A bispecific anti-PLAP/GCAP anti-mouse CD3 antibody (Ab) 7E8 x 7D6, previously shown to induce efficient dose-dependent T-cell proliferation and PLAP+ tumor cell lysis in the presence of recombinant IL-2 and the anti-mouse CD3 Ab 7D6, was used in this report in in vivo lysis experiments targeting GCAP+ tumors grown in GCAP+ transgenic mice. Mice received injections i.v. twice a week with PBS (group 1) or with 10 micrograms of the bispecific Ab 7E8 x 7D6, either alone (group 2) or combined with 1 microgram of the anti-CD3 Ab 7D6 (group 3), starting 7 days after the tumor inoculation. A fourth group received a local treatment with mouse splenocytes precoated with 10 micrograms 7E8 x 7D6 and 1 microgram 7D6. In between Ab injections, groups 2, 3, and 4 received 10(4) units recombinant IL-2 (i.v.) every day. Two weeks of treatment with the bispecific Ab either alone or combined with 7D6 resulted in a significant decrease of GCAP+ tumor cells in groups 2 and 3 (4 +/- 3% and 10 +/- 11% GCAP+ cells/tumor) as compared to the nontreated tumors (95 +/- 5% GCAP+ cells), although tumor volumes were not significantly different (12 +/- 15 cm3 and 14 +/- 11 cm3 versus 16 +/- 7 cm3). Apparently, the elimination of GCAP+ cells from the tumor seemed to favor conditions enabling the outgrowth of the few GCAP- cells originally present in the tumor inoculate. In contrast, tumor volumes in group 4 (local treatment) were significantly smaller (P < 0.03; 5 +/- 10 cm3, 8 +/- 11% GCAP+ cells) as compared to the nontreated group, probably due to the presence of higher amounts of Ab and infiltrated activated T cells (567 +/- 322 CD5+ cells/mm2) capable of secreting cytostatic cytokines like tumor necrosis factor alpha and IFN-gamma as compared to groups 2 and 3 (266 +/- 135 and 198 +/- 86 CD5+ cells/mm2, respectively). In summary, this study clearly demonstrated that bispecific antibodies specifically concentrate cytotoxic T cells into a solid tumor in vivo, with subsequent elimination of the targeted tumor cell.

Tissue-nonspecific alkaline phosphatase participates in the establishment and growth of feather germs in embryonic chick skin cultures

Alkaline phosphatase activity is present in the mesoderm of embryonic chick skin and becomes spatially restricted to the dermal condensation of the developing feather germs. Inhibitors to tissue-nonspecific (liver/bone/kidney), but not intestinal alkaline phosphatase inhibit the establishment and growth of feather germs in cultured skins. A window of maximum sensitivity to the inhibitor was observed to be the first day of culture when early development and establishment of pattern takes place. The cDNA for the avian tissue-nonspecific alkaline phosphatase was cloned and sequenced, and Southern analysis revealed a single copy of this gene in the avian genome. Northern analysis revealed that a 2.8 kb transcript for this form of alkaline phosphatase is present in developing skin.

Biology of human alkaline phosphatases with special reference to cancer

The current information on the cloning and sequencing of four alkaline phosphatase genes (PLAP, GCAP, IAP, TNAP) has been reviewed. It has provided insights into their evolutionary history and the mechanisms of catalysis and of uncompetitive inhibition. The oncodevelopmental biology of the germ cell and its excessive GCAP eutopic expression in neoplasia are noted, and there is reason to suggest that the enzyme may serve to guide migratory cells and to transport specific molecules such as fat and immunoglobulins across membranes. The hyperexpression of all four genes has been observed in various human tumors and in their cell lines, particularly cancers of the testis and ovary. The membrane APs have been investigated as targets for immunolocalization and immunotherapy.

A haploid and a diploid cell coexist in an in vitro immortalized spermatogenic cell line

We have recently established a conditionally immortalized germ cell line [GC-2spd(ts)] that, at the permissive temperatures of 37 degrees C and 32 degrees C, is able to undergo meiosis in vitro and form round spermatids [Hofmann et al., (1994): Proc Natl Acad Sci USA 91:5533-5537]. In this report, we provide data that indicate that the GC-2spd(ts) cell line consists of two cell populations undergoing a haploid (n/2n) cell cycle and a diploid (2n/4n) cell cycle, respectively. The cells containing 2n DNA, when sorted by fluorescence-activated cell sorting, are able to reconstitute the full population of n/2n/4n DNA cells, indicating that they are able to commit to the reductive meiotic division and form haploid spermatids or to continue self-replication through a diploid cell cycle. This GC-2spd(ts) cell line provides a valuable tool to study the molecular mechanisms involved in the cellular decision between self-renewal by mitosis and commitment to meiosis.

Stage-specific expression of alkaline phosphatase during neural development in the mouse

The expression pattern of tissue nonspecific alkaline phosphatase (TNAP) in the developing neural tube of mouse is reported. Homogeneous AP activity in the neuroepithelium becomes prominent at E8.5. At E9.5, distinctly AP-positive cells appear in the brain and spinal cord area. At stages E10.5 to E12.5, AP positivity is observed between the mesencephalon and the rhombencephalon, along the entire spinal cord and cranial nerves emerging from the myelencephalon. At E13.5, strongly AP positive fibers become prominent in the pons. At E14.5, AP expression in brain tissue is considerably reduced and there is a complete absence of AP activity in the nerve cells and glial cells of adult brain. The choroid plexus remains distinctly positive for AP expression until the adult stage. Northern blot analysis and reverse-transcriptase polymerase chain reaction amplification of RNA indicate that this AP activity results from the expression of the Akp-2 locus. This AP expression pattern is distinct from those reported for the expression of GD3, nestin, Hox 2.3, and Wnt-1 during brain development. We conclude that AP is a useful marker of a subpopulation of neuroectodermal cells present in the neural tube as early as E8.5, at which stages there are no other AP positive intraembryonic cells except PGCs.

Conversion of secretory proteins into membrane proteins by fusing with a glycosylphosphatidylinositol anchor signal of alkaline phosphatase

Placental alkaline phosphatase (PLAP) is initially synthesized as a precursor (proPLAP) with a C-terminal extension. We constructed a recombinant cDNA which encodes a chimeric protein (alpha GL-PLAP) comprising rat alpha 2u-globulin (alpha GL) and the C-terminal extension of PLAP. Two molecular species (25 kDa and 22 kDa) were expressed in the COS-1 cell transfected with the cDNA for alpha GL-PLAP. Only the 22 kDa form was labelled with both [3H]stearic acid and [3H]ethanolamine. Upon digestion with phosphatidylinositol-specific phospholipase C the 22 kDa form was released into the medium, indicating that this form is anchored on the cell surface via glycosylphosphatidylinositol (GPI). A specific IgG raised against a C-terminal nonapeptide of proPLAP precipitated the 25 kDa form but not the 22 kDa form, suggesting that the 25 kDa form is a precursor retaining the C-terminal propeptide. When a mutant alpha GL-PLAP, in which the aspartic acid residue is replaced with tryptophan at a putative cleavage/attachment site, was expressed in COS-1 cells, the 25 kDa precursor was the only form found inside the cell and retained in the endoplasmic reticulum, as judged by immunofluorescence microscopy. In vitro translation programmed with mRNAs coding for the wild-type and mutant forms of alpha GL-PLAP demonstrated that the C-terminal propeptide was cleaved from the wild-type chimeric protein, but not from the mutant one. This gave rise to the 22 kDa form attached with a GPI anchor, suggesting that GPI is covalently linked to the aspartic acid residue (Asp159) of alpha GL-PLAP. Taken together, these results indicate that the C-terminal propeptide of PLAP functions as a signal to render alpha GL a GPI-linked membrane protein in vitro and in vivo in cultured cells, and that the chimeric protein constructed in this study may be useful for elucidating the mechanism underlying the cleavage of the propeptide and attachment of GPI, which occur in the endoplasmic reticulum.

Ethnic differences in enzyme and DNA polymorphisms of human tissue-specific alkaline phosphatases

Enzyme and DNA polymorphisms and haplotypes of tissue-specific alkaline phosphatase genes were studied in Finns, Saamis and Swedes. In placental alkaline phosphatase (PLAP) restriction fragment length polymorphisms (RFLPs), found after digestion with RsaI and PstI, no significant population differences were observed. The PstI RFLP of the germ cell alkaline phosphatase (GCAP) locus showed significant allele frequency variations between Finns and Swedes (p = 0.014) and between Saamis and Swedes (p = 1 x 10(-4)). Electrophoretic enzyme variants of PLAP were studied in Finns and Swedes. The PLAP variant 18 (D in older nomenclature) was found at a polymorphic frequency in the Finnish sample. In all populations, strong linkage disequilibria were found between the RFLPs and between RFLPs and electrophoretic PLAP types. There was, however, one notable exception: between the RsaI RFLP of PLAP and the PstI RFLP of GCAP no linkage disequilibrium was found. Two new RFLPs were observed in the Finnish population sample, a RsaI mutant site in PLAP with a frequency of 0.02 and a KpnI mutant site outside and upstream of the PLAP gene with a frequency of 0.036. In accordance with findings in previous studies at the enzyme level, PLAP also appeared to be more polymorphic than GCAP and intestinal alkaline phosphatase at the DNA level.

Immortalized germ cells undergo meiosis in vitro

Establishing mammalian germ-cell lines capable of differentiation in vitro would greatly facilitate the study of gametogenesis and the meiotic process that is so fundamental for reproduction and the maintenance of genetic diversity of the species. We have established two germ-cell lines [GC-2spd(ts) and GC-3spc(ts)] by cotransfecting primary mouse testicular germ cells with the simian virus 40 large tumor antigen gene and the gene coding for a temperature-sensitive mutant of p53. Both cell lines express the germ cell-specific lactate dehydrogenase C4 isozyme and cytochrome ct isoform. At the permissive temperature of 37 degrees C, the GC-2spd(ts) line generates cells with a haploid DNA content and morphologic and biochemical features of round spermatids, including the appearance of an acrosomic granule. The identification of a flagellar axoneme when these cells are cultured at 32 degrees C further indicates that these cells correspond to the early spermatid stages of spermiogenesis.

PstI restriction fragment length polymorphism of the human intestinal alkaline phosphatase gene

Restriction fragment length polymorphisms have previously been found in the placental alkaline phosphatase (PLAP) and germ cell alkaline phosphatase (GCAP) genes, but not in the closely related intestinal alkaline phosphatase (IAP) locus. We here report on a PstI restriction fragment length polymorphism in IAP found in Finns and Swedes but not in Saamis. A probable T-->G mutation in position 175 of intron 11 would create a new cleavage site for PstI. The borderline frequency of the mutant allele (0.01) is in agreement with previous observations suggesting that IAP is considerably less polymorphic than PLAP and GCAP.

Modifications in a flexible surface loop modulate the isozyme-specific properties of mammalian alkaline phosphatases

We have analyzed to what extent the surface loop domain of alkaline phosphatases (APs) is responsible for isozyme-specific functional properties. Unique AatII and RsrII restriction sites were introduced by site-directed mutagenesis at identical positions in murine tissue-nonspecific AP (TNAP) and human placental AP (PLAP) cDNAs to allow the homologous exchange of the loop domain of the TNAP (T domain) and PLAP (P domain) isozymes and the generation of the reciprocally chimeric molecules PLAP-T and TNAP-P. The introduction of the T loop into PLAP reduced the heat stability of PLAP-T to almost that of TNAP. The domain substitution was accompanied by a conformational change that resulted in the loss of immune reactivity with four of 17 epitope-mapped anti-PLAP monoclonal antibodies. The T and P loops provided stabilization to the side chain of specific uncompetitive AP inhibitors. The introduction of the T domain also conferred collagen-binding properties to PLAP-T accounting for half of the binding affinity of TNAP for collagen, while not affecting PLAP binding to IgG. Our data indicate that the surface loop determines overall enzyme stability, differs conformationally in the various isozymes, and modulates catalytic parameters in the presence of protein ligands, thus, accounting in part for isozyme-specific protein interactions.

Transgenic mice expressing the tumor marker germ cell alkaline phosphatase: an in vivo tumor model for human cancer antigens

We have generated a series of transgenic mouse lines harboring the entire human germ cell alkaline phosphatase (GCAP) gene linked to progressively longer sequences of flanking DNA. A 450-bp promoter sequence directs the expression of GCAP to the intestine and endothelial cells, while a 5' sequence of 1.7 kb directs GCAP expression to the spermatogenic lineage and to the eight-cell through the blastocyst stage of preimplantation development. The expression of GCAP in these FVB/N transgenic mice induces a cellular immune tolerance to GCAP. When mouse fibrosarcoma MO4 cells (C3H derived), stably transfected with the cloned GCAP gene, were injected s.c. in nontransgenic control (C3H x FVB/N) hybrid mice, GCAP-positive tumor cells were rejected. However, when GCAP-expressing transgenic (C3H x FVB/N) hybrid mice were challenged with these cells, GCAP-positive tumors developed. Tumors also developed in the transgenic hybrid mice upon injection of MO4 cells transfected with the highly homologous placental alkaline phosphatase (PLAP) cDNA in spite of the presence in PLAP of 10 amino acids that are different from the corresponding residues in GCAP. These GCAP transgenic mice will allow the study of the immune response associated with the repeated administration of conjugated or derivatized anti-GCAP and anti-PLAP monoclonal antibodies. They will also enable evaluation of the therapeutic potential of bifunctional antibodies for T-cell recruitment and destruction of GCAP/PLAP-producing tumor cells.

Genomic structure and promoter activity of the human testis lactate dehydrogenase gene

The structure of the gene encoding the human testis-specific isozyme of lactate dehydrogenase (LDH) has been characterized and a regulatory region identified by promoter activity. The single-copy ldh-c gene has two alternative 5' noncoding exons and seven coding exons comprising an approximately 40-kb locus. The gene does not contain the canonical TATA or CAAT promoter sequences, and ribonuclease protection experiments suggest multiple transcription start sites. In the present study an immortalized murine germ cell line was used to detect promoter activity driven by 5' sequence of human ldh-c with lacZ as the reporter gene. Reporter gene activity was nondetectable when promoter constructs were transfected into nongerminal cells.