Mapping of gene encoding mouse placental alkaline phosphatase to chromosome 4

Tissue-nonspecific Alkaline Phosphatase Regulates Purinergic Transmission in the Central Nervous System During Development and Disease

Tissue-nonspecific alkaline phosphatase (TNAP) is one of the four isozymes in humans and mice that have the capacity to hydrolyze phosphate groups from a wide spectrum of physiological substrates. Among these, TNAP degrades substrates implicated in neurotransmission. Transgenic mice lacking TNAP activity display the characteristic skeletal and dental phenotype of infantile hypophosphatasia, as well as spontaneous epileptic seizures and die around 10 days after birth. This physiopathology, linked to the expression pattern of TNAP in the central nervous system (CNS) during embryonic stages, suggests an important role for TNAP in neuronal development and synaptic function, situating it as a good target to be explored for the treatment of neurological diseases. In this review, we will focus mainly on the role that TNAP plays as an ectonucleotidase in CNS regulating the levels of extracellular ATP and consequently purinergic signaling.

Multisystemic functions of alkaline phosphatases

Human and mouse alkaline phosphatases (AP) are encoded by a multigene family expressed ubiquitously in multiple tissues. Gene knockout (KO) findings have helped define some of the precise exocytic functions of individual isozymes in bone, teeth, the central nervous system, and in the gut. For instance, deficiency in tissue-nonspecific alkaline phosphatase (TNAP) in mice (Alpl (-/-) mice) and humans leads to hypophosphatasia (HPP), an inborn error of metabolism characterized by epileptic seizures in the most severe cases, caused by abnormal metabolism of pyridoxal-5'-phosphate (the predominant form of vitamin B6) and by hypomineralization of the skeleton and teeth featuring rickets and early loss of teeth in children or osteomalacia and dental problems in adults caused by accumulation of inorganic pyrophosphate (PPi). Enzyme replacement therapy with mineral-targeting TNAP prevented all the manifestations of HPP in mice, and clinical trials with this protein therapeutic are showing promising results in rescuing life-threatening HPP in infants. Conversely, TNAP induction in the vasculature during generalized arterial calcification of infancy (GACI), type II diabetes, obesity, and aging can cause medial vascular calcification. TNAP inhibitors, discussed extensively in this book, are in development to prevent pathological arterial calcification. The brush border enzyme intestinal alkaline phosphatase (IAP) plays an important role in fatty acid (FA) absorption, in protecting gut barrier function, and in determining the composition of the gut microbiota via its ability to dephosphorylate lipopolysaccharide (LPS). Knockout mice (Akp3 (-/-)) deficient in duodenal-specific IAP (dIAP) become obese, and develop hyperlipidemia and hepatic steatosis when fed a high-fat diet (HFD). These changes are accompanied by upregulation in the jejunal-ileal expression of the Akp6 IAP isozyme (global IAP, or gIAP) and concomitant upregulation of FAT/CD36, a phosphorylated fatty acid translocase thought to play a role in facilitating the transport of long-chain fatty acids into cells. gIAP, but not dIAP, is able to modulate the phosphorylation status of FAT/CD36. dIAP, even though it is expressed in the duodenum, is shed into the gut lumen and is active in LPS dephosphorylation throughout the gut lumen and in the feces. Akp3 (-/-) mice display gut dysbiosis and are more prone to dextran sodium sulfate-induced colitis than wild-type mice. Of relevance, oral administration of recombinant calf IAP prevents the dysbiosis and protects the gut from chronic colitis. Analogous to the role of IAP in the gut, TNAP expression in the liver may have a proactive role from bacterial endotoxin insult. Finally, more recent studies suggest that neuronal death in Alzheimer's disease may also be associated with TNAP function on certain brain-specific phosphoproteins. This review recounts the established roles of TNAP and IAP and briefly discusses new areas of investigation related to multisystemic functions of these isozymes.

A novel phosphatase upregulated in Akp3 knockout mice

Reexamination of the Akp3(-/-) mouse intestine showed that, despite the lack of intestinal alkaline phosphatase (IAP), the Akp3(-/-) gut still had considerable alkaline phosphatase (AP) activity in the duodenum and ileum. This activity is due to the expression of a novel murine Akp6 gene that encodes an IAP isozyme expressed in the gut in a global manner (gIAP) as opposed to duodenum-specific IAP (dIAP) isozyme encoded by the Akp3 gene. Phylogenetically, gIAP is similar to the rat IAP I isozyme. Kinetically, gIAP displays a 5.7-fold reduction in catalytic rate constant (k(cat)) and a 30% drop in K(m), leading to a 4-fold reduction k(cat)/K(m) compared with dIAP, and these changes in enzymatic properties can all be attributed to a crucial R317Q substitution. Western and Northern blot analyses document the expression of Akp6 in the gut, from the duodenum to the ileum, and it is upregulated in the jejunum and ileum of Akp3(-/-) mice. Developmentally, Akp3 expression is turned on during postnatal days 13-15 and exclusively in the duodenum, whereas Akp6 and Akp5 are expressed from birth throughout the gut with enhanced expression at weaning. Posttranslational modifications of gIAP have a pronounced effect on its catalytic properties. Given the low catalytic efficiency of gIAP, its upregulation during fat feeding, its sequence similarity with rat IAP I, and the fact that rat IAP I has been implicated in the upregulation of surfactant-like particles during fat intake, it appears likely that gIAP may have a role in mediating the accelerated fatty acid intake observed in Akp3(-/-) mice fed a high-fat diet.

A new variant of the mouse Akp1 locus

A new electrophoretic migration type of alkaline phosphatase 1 (Akp1) was found on the cellulose acetate electrophoresis for kidney and liver homogenates of KWHM mouse, a newly established inbred strain derived from the Korean wild mouse (Mus musculus molossinus). This new type of alkaline phosphatase 1 was distinguished from previously reported AKP1A and AKP1B types in the mouse, and tentatively named AKP1C. In genetic analysis by mating experiments between KWHM and C57BL/6J (AKP1A) or BALB/cA (AKP1B), the phenotypic segregation ratios of AKP1A : AKP1AC : AKP1C or AKP1B : AKP1BC : AKP1C were 1 : 2 : 1 in both groups of F2 generations. It was therefore concluded that AKP1C type is controlled by Akp1c allele which is codominant with Akp1a and Akp1b alleles.

Serum alkaline phosphatase activity is regulated by a chromosomal region containing the alkaline phosphatase 2 gene (Akp2) in C57BL/6J and DBA/2J mice

Quantitative trait locus (QTL) analyses were conducted to identify chromosomal regions that contribute to variability in serum alkaline phosphatase (AP) enzyme activity in mice derived from the C57BL/6J (B6) and DBA/2J (D2) inbred strains. Serum AP was measured in 400 B6D2 F2 mice at 5 mo and 400 B6D2 F2 mice at 15 mo of age that were genotyped at 96 microsatellite markers, and in 19 BXD recombinant inbred (RI) strains at 5 mo of age. A QTL on the distal end of chromosome 4 was present in all sex- and age-specific analyses with a peak logarithm of odds (LOD) score of 20.36 at 58.51 cM. The Akp2 gene, which encodes the major serum AP isozyme, falls within this QTL region at 70.2 cM where the LOD score reached 13.2 (LOD significance level set at 4.3). Serum AP activity was directly related to the number of D2 alleles of a single nucleotide polymorphism in the 5'-flanking region of the Akp2 gene, although no strain-related differences in hepatic expression of Akp2 RNA were found. A variety of sequence polymorphisms in this chromosomal region could be responsible for the differences in serum AP activity; the Akp2 gene, however, with several known amino acid substitutions between protein sequences of the B6 and D2 strains, is a leading candidate.

Retinoic acid and methylation cis-regulatory elements control the mouse tissue non-specific alkaline phosphatase gene expression

To understand the mechanisms regulating the tissue non-specific alkaline phosphatase (TNAP) activity during development, we characterized cis-transcriptional regulatory elements. In embryonic cells and tissues, TNAP expression was driven preferentially by the exon 1A (E1A) promoter, one of the two promoters previously defined. Transcriptional activity of E1A promoter was up-regulated by retinoic acid (RA) through a putative RA-responsive element. Transgenic mice analysis with lacZ reporter constructs revealed negative regulatory elements within 8.5 kb of E1A promoter. Promoter sequences of endogenous TNAP in non-expressing tissues and those carried by the 8.5 kb-lacZ transgene were found to be highly methylated. A 1 kb fragment of E1A promoter increased the methylation level of lacZ and promoter sequences. The role of RA and DNA methylation in defining the embryonic expression pattern of TNAP is discussed.

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.

Expression of alkaline phosphatase in murine lymphoma cells

Alkaline phosphatase (ALP) was secreted and expressed at the cell surface of the lymphoma A/63-2 cell line but not on another clone A/63-1 deriving from a single thymoma (A/63) induced by a wild-type Abelson-Moloney viral complex. The enzyme was heat-sensitive and strongly inhibited by L-p-bromotetramisole and L-homoarginine but not by L-phenylalanine. All these data indicated that this enzyme was most likely identical to the L/B/K ALP isoenzyme. Southern blot analysis showed that neither amplification nor polymorphism were responsible for the high expression of the ALP gene observed in A/63-2 cells. On the opposite, the mRNA transcripts of ALP were only detected in A/63-2 cells indicating that a modulation of the ALP gene transcription occurred which could be due to the insertion of the v-abl gene within or near the 5'-flanking region of the ALP promotor in A/63-2 cells. Butyrate strongly increased both the secretion and the expression of the enzyme on A/63-2 cell surface. This induction was strongly inhibited by cordycepin, an RNA biosynthesis inhibitor, and at a lesser degree by cycloheximide, a translation inhibitor suggesting that butyrate induction occurs both at the transcriptional and the translational level.

Intestinal leucine aminopeptidase and alkaline phosphatase: genetic regulation and development in mice

Intestinal and serum leucine aminopeptidase (LAP) and alkaline phosphatase (AKP) were characterized by electrophoresis for eight inbred strains of laboratory mice. Intestinal LAP and AKP of adult mice were expressed concordantly within strains, as banded or diffuse, and concordantly for rate of migration within strains that had diffuse isozymes. All strains, except DD/S, had a single band of serum LAP and a single, diffuse zone of serum AKP. DD/S had a double band of serum LAP as well as isozymes of intestinal LAP and AKP unlike those of other strains. All strains displayed similar, neuraminidase-sensitive isozymes of intestinal LAP and of AKP prior to weaning, but after weaning there was marked sensitivity to neuraminidase only in DD/S. In interstrain crosses, banded/diffuse, migration rate, and neuraminidase sensitivity were inherited as independent autosomal traits, with indications of variable penetrance and genetic interaction.

Rapid analysis of mouse-hamster hybrid cell lines by in situ hybridization

In situ hybridization techniques for analyzing the murine DNA complement of mouse-hamster hybrid cells are described. Total genomic mouse DNA is labeled with biotin and hybridized without suppression to metaphase spreads from a mouse-hamster hybrid line containing the mouse fusion chromosome X12. Detection via fluorochrome-conjugated avidin reveals mouse chromosomal DNA with high sensitivity and permits the identification of both normal and aberrant murine chromosomes. Conversely, biotinylated total genomic DNA from a hybrid line can be used as a probe on normal mouse metaphase spreads if suppression techniques are employed, facilitating the analysis of mouse chromosomes present in the hybrid line.

Identity of PB76 differentiation antigen and lymphocyte alkaline phosphatase

Alkaline phosphatases (APases, EC 3.1.3.1) are ecto-enzymes bound to cell membranes by a phosphatidyl-inositol anchor. We have previously shown that APase is present on activated murine B cells and its expression correlates with the process of B cell differentiation into immunoglobulin secretion. Recently, a monoclonal antibody (mAb), G-5-2, that recognizes a 76-kDa molecule preferentially expressed on the surface of pre-B and plasma cells (PB76) was described. Some features shared by APase and PB76 differentiation antigen suggest that the G-5-2 mAb might be specific for lymphocyte APase. Here, we have analyzed this possibility and found an absolute correlation between PB76 expression in cells and their APase activity. Although PB76 has been described as a B cell-restricted marker, PB76 is also expressed on some T cells, such as the YAC-1 T cell lymphoma, that are known to bear APase. Treatment of YAC-1 cells with phosphatidylinositol-specific phospholipase C resulted in a quantitatively correlated removal of both APase and PB76 antigens. Moreover, we demonstrate that PB76 antigen has APase activity using an enzyme-antigen immunoassay with the G-5-2 mAb. We conclude that PB76 and lymphocyte APase are one and the same antigen.

Chromosome maps of man and mouse. IV

Current knowledge of man-mouse genetic homology is presented in the form of chromosomal displays, tables and a grid, which show locations of the 322 loci now assigned to chromosomes in both species, as well as 12 DNA segments not yet associated with gene loci. At least 50 conserved autosomal segments with two or more loci have been identified, twelve of which are over 20 cM long in the mouse, as well as five conserved segments on the X chromosome. All human and mouse chromosomes now have conserved regions; human 17 still shows the least evidence of rearrangement, with a single long conserved segment which apparently spans the centromere. The loci include 102 which are known to be associated with human hereditary disease; these are listed separately. Human parental effects which may well be the result of genomic imprinting are reviewed and the location of the factors concerned displayed in relation to mouse chromosomal regions which have been implicated in imprinting phenomena.

Mouse Ly-31.1 is an alloantigenic determinant of alkaline phosphatase predominantly expressed in the kidney and bone

The mouse lymphocyte surface alloantigen, Ly-31, defined by monoclonal antibody N1.10 (IgG2b,k) and controlled by a gene locus closely linked to the Akp-2 locus on chromosome 4, was biochemically investigated. By employing a quantitative immunoassay system, it was found that the Ly-31.1-specific antibody detected an allotypic determinant of mouse alkaline phosphatase. Ly-31.1, i.e., mouse alkaline phosphatase, was expressed predominantly in kidney and bone and was also detected in placenta, lung, and testis. Concerning tumor cell lines, they varied in the amount of antigen present, with both T and B lymphoid lineages selectively possessing the antigen. In normal lymphoid tissues, lesser amounts of antigen were detected. The binding of mouse alkaline phosphatase to Ly-31.1-specific monoclonal antibodies was specific in nature. The Ly-31.1 antigen was immunoprecipitated from the lysates of surface-radiolabeled YAC-1 moloney leukemia cells, and appeared as a single band of about 78,000 under both reduced and nonreduced conditions on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Furthermore, treatment of tumor cell lines with phosphatidylinositol-specific-phospholipase C resulted in the removal of Ly-31 antigen from the cell surface. These results suggest that a gene cluster containing the Ly-31 and Akp-2 loci which control the alkaline phosphatase is formed on mouse chromosome 4. The Ly-31 antigen is the first enzyme demonstrated to be a lymphocyte surface alloantigen.