Muscular dystrophy and neuronal migration disorder caused by mutations in a glycosyltransferase, POMGnT1.

Muscle-eye-brain disease (MEB) is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities, and lissencephaly. Mammalian O-mannosyl glycosylation is a rare type of protein modification that is observed in a limited number of glycoproteins of brain, nerve, and skeletal muscle. Here we isolated a human cDNA for protein O-mannose beta-1,2-N-acetylglucosaminyltransferase (POMGnT1), which participates in O-mannosyl glycan synthesis. We also identified six independent mutations of the POMGnT1 gene in six patients with MEB. Expression of most frequent mutation revealed a great loss of the enzymatic activity. These findings suggest that interference in O-mannosyl glycosylation is a new pathomechanism for muscular dystrophy as well as neuronal migration disorder.

Function and Change with Aging of α-Klotho in the Kidney.

The α-Klotho mouse is an animal model that prematurely shows phenotypes resembling human aging, such as osteoporosis, arteriosclerosis, pulmonary emphysema, and kidney damage. Interestingly, these abnormalities are triggered by a deficiency of a single protein, α-Klotho. The kidney is an organ that highly expresses α-Klotho, suggesting that α-Klotho is important for kidney function. Recent studies suggest that α-Klotho is associated with phosphate, vitamin D, and calcium homeostasis. The calcium imbalance in α-Klotho mice may induce calpain overactivation, leading to cell death and tissue destruction. α-Klotho is predicted to have glycosidase activity, capable of modifying the N-glycans of channels and transporters and regulating transmembrane movement of several ions, including calcium. Interestingly, N-glycan changes are observed in the kidney of α-Klotho mice and normal aged mice in association with decreased α-Klotho levels. These results imply that glycobiology and α-Klotho function are interesting targets for future studies.

Molecular cloning of cDNAs encoding alpha1, alpha2, and beta subunits of rat brain platelet-activating factor acetylhydrolase.

Brain intracellular platelet-activating factor acetylhydrolase (PAF-AH(Ib)) is a tertiary G-protein-complex-like heterotrimeric enzyme which is composed of alpha1, alpha2, and beta subunits and is implicated in stages of brain development such as the formation of the brain cortex. We have isolated and sequenced cDNA clones encoding these three subunits of rat brain PAF-AH(Ib). The amino acid sequences of brain PAF-AH has shown an extremely high homology among mammalian species. The tissue distribution of the three subunits was examined by Northern blot analysis. Although the mRNAs were detected in various organs, the ratio of the level of mRNA expression for the three subunits differed among rat tissues, raising the possibility that isoform(s) other than the heterotrimeric isoform exist in certain tissues.

Fukutin and alpha-dystroglycanopathies.

Fukuyama-type congenital muscular dystrophy (FCMD), Walker-Warburg syndrome (WWS), and muscle-eye-brain (MEB) disease are clinically similar autosomal recessive disorders characterized by congenital muscular dystrophy, lissencephaly, and eye anomalies. We identified the gene for FCMD and MEB, which encodes the fukutin protein and the protein O-linked mannose beta1, 2-N-acetylglucosaminyltransferase (POMGnT1), respectively. Recent studies have revealed that posttranslational modification of alpha-dystroglycan is associated with these congenital muscular dystrophies with brain malformations. All are characterized by hypoglycosylated alpha-dystroglycan. Fukutin's function and the relation with other alpha-dystroglycanopathies are discussed.

Induction of terminal differentiation and apoptosis in human colonic carcinoma cells by brefeldin A, a drug affecting ganglioside biosynthesis.

An appreciable increase in G(M3) with a concomitant decrease in some neolacto-series gangliosides was observed during differentiation of human colonic carcinoma HCT 116 cells induced by a differentiating agent. When the cells were treated with brefeldin A (BFA), a striking increase in de novo biosynthesis of G(M3) and a decrease in biosynthesis of neolactoseries gangliosides were observed after 6 h. Clear morphological changes to differentiated epithelial cells and an arrest of cells in the G0/G1 phase of the cell cycle were observed after 1 day of treatment. Then the cells were led to apoptosis. This activity was not affected by forskolin, which antagonizes the effects of BFA on protein transport and the Golgi apparatus. These results suggest that the differentiation-inducing activity of BFA might be due to its modulatory effect on ganglioside biosynthesis, and that a specific change in ganglioside pattern is an essential prerequisite for induction of differentiation, providing a novel target for differentiation therapy of cancer.

Comparative study of the asparagine-linked sugar chains of human lipocalin-type prostaglandin D synthase purified from urine and amniotic fluid, and recombinantly expressed in Chinese hamster ovary cells.

Lipocalin-type prostaglandin D synthase (L-PGDS) is a highly glycosylated member of the lipocalin gene family and is secreted into various human body fluids. We comparatively analyzed the structures of asparagine-linked sugar chains of human L-PGDS produced by recombinant Chinese hamster ovary cells and naturally occurring human urine and amniotic fluid. After the sugar chains were liberated by hydrazinolysis followed by N-acetylation, they were derivatized with 2-aminobenzamide. All of the sugar chains of three L-PGDSs occur as biantennary complex-type sugar chains. Most of the sugar chains of three samples were fucosylated on the inner most N-acetylglucosamine residue. Although the sugar chains of the recombinant L-PGDS do not contain any bisecting N-acetylglucosamine residues, 58% and 34% of the fucosylated-sugar chains of amniotic fluid and urine L-PGDSs, respectively, contain bisecting N-acetylglucosamine residues. The sialic acid residues occur solely as Siaalpha2-->3Gal groups of the recombinant L-PGDS; the sialic acid residues of other L-PGDS occur as both Siaalpha2-->3Gal and Siaalpha2-->6Gal groups. Variations in L-PGDS glycosylation may prove useful as markers to further elucidate the role of L-PGDS glycoforms in different tissues.

PAF analogues capable of inhibiting PAF acetylhydrolase activity suppress migration of isolated rat cerebellar granule cells.

Intracellular platelet-activating factor (PAF) acetylhydrolase in the bovine brain is a heterotrimeric enzyme composed of alpha1, alpha2 and beta subunits. The trimeric enzyme may be involved in neural cell migration, since the human homolog of the non-catalytic beta subunit is a product of the LIS-1 gene which is a causative gene for Miller-Dieker syndrome. Miller-Dieker syndrome is a form of lissencephaly that is characterized by widespread agyria of the brain and defects of neuronal cell migration. In the present study, we attempted to determine whether the catalytic activity of either the alpha1 or alpha2 subunit is required for the regulation of granule cell migration. Granule cells prepared from rat cerebellum at postnatal day 0 express all three subunit proteins (alpha1, alpha2 and beta) as determined by western blotting. Granule cell migration, which was observed in vitro on a layer coated with laminin, was effectively blocked by PAF analogs which showed PAF receptor-antagonistic activity (CV-6209 and CV-3988) and PAF receptor-agonistic activity (carbamoyl PAF). These PAF analogs also inhibited the activity of bovine brain PAF acetylhydrolase. Cell migration was restored when the inhibitors were removed by washing the treated cells with buffer, indicating that the inhibitory effect of PAF analogs is reversible. Structurally-unrelated PAF antagonists (SM-12502, TCV-309 and YM-264), none of which showed any appreciable inhibitory activity against PAF acetylhydrolase, did not block granule cell migration under the same conditions. It is suggested that the catalytic activity of PAF acetylhydrolase may play a crucial role in neural cell migration.

Biochemical characterization of various catalytic complexes of the brain platelet-activating factor acetylhydrolase.

Brain intracellular platelet-activating factor acetylhydrolase (PAF-AH) isoform I is a member of a family of complex enzymes composed of mutually homologous alpha(1) and alpha(2) subunits, both of which account for catalytic activity, and the beta subunit. We previously demonstrated that the expression of one catalytic subunit, alpha(1), is developmentally regulated, resulting in a switching of the catalytic complex from alpha(1)/alpha(2) to alpha(2)/alpha(2) during brain development (Manya, H., Aoki, J., Watanabe, M., Adachi, T., Asou, H., Inoue, Y., Arai, H., and Inoue, K. (1998) J. Biol. Chem. 273, 18567-18572). In this study, we explored the biochemical differences in three possible catalytic dimers, alpha(1)/alpha(1), alpha(1)/alpha(2), and alpha(2)/alpha(2). The alpha(2)/alpha(2) homodimer exhibited different substrate specificity from the alpha(1)/alpha(1) homodimer and the alpha(1)/alpha(2) heterodimer, both of which showed similar substrate specificity. The alpha(2)/alpha(2) homodimer hydrolyzed PAF and 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylethanolamine (AAGPE) most efficiently among 1-O-alkyl-2-acetyl-phospholipids. In contrast, both alpha(1)/alpha(1) and alpha(1)/alpha(2) hydrolyzed 1-O-alkyl-2-acetyl-sn-glycero-3-phosphoric acid more efficiently than PAF. AAGPE was the poorest substrate for these enzymes. The beta subunit bound to all three catalytic dimers but modulated the enzyme activity in a catalytic dimer composition-dependent manner. The beta subunit strongly accelerated the enzyme activity of the alpha(2)/alpha(2) homodimer but rather suppressed the activity of the alpha(1)/alpha(1) homodimer and had little effect on that of the alpha(1)/alpha(2) heterodimer. The (His(149) to Arg) mutant beta, which has been recently identified in isolated lissencephaly sequence patients, lost the ability to either associate with the catalytic complexes or modulate their enzyme activity. The enzyme activity of PAF-AH isoform I may be regulated in multiple ways by switching the composition of the catalytic subunit and by manipulating the beta subunit.

Switching of platelet-activating factor acetylhydrolase catalytic subunits in developing rat brain.

In a previous study, we demonstrated that Platelet-activating Factor (PAF) acetylhydrolase purified from bovine brain cortical cytosol consists of two mutually homologous catalytic subunits (alpha1 and alpha2) and one putative regulatory beta subunit. The latter is a product of the LIS1 gene, which is defective in the Miller-Dieker syndrome, a form of lissencephaly. In this study, we examined the expression patterns of these three subunits in the developing rat brain. All three subunits were expressed in embryonic brain, whereas only alpha2 and beta subunit were detected in the adult brain by Western blotting. Biochemical analyses revealed that the alpha1/alpha2 heterodimer and alpha2/alpha2 homodimer are major catalytic units of embryonic and adult brain PAF acetylhydrolases, respectively. The alpha1 transcript and protein were detected predominantly in embryonic and postnatal neural tissues, such as the brain and spinal cord. Furthermore, we found using primary cultured cells isolated from neonatal rat brain that alpha1 protein were expressed only in neurons but not in glial cells and fibroblasts. In contrast, alpha2 and beta transcripts and proteins were detected both in neural and non-neural tissues, and their expression level was almost constant from fetal stages through adulthood. These results indicate that alpha1 expression is restricted to actively migrating neurons in rats and that switching of catalytic subunits from the alpha1/alpha2 heterodimer to the alpha2/alpha2 homodimer occurred in these cells during brain development, suggesting that PAF acetylhydrolase plays a role(s) in neuronal migration.

A new beta-1,2-N-acetylglucosaminyltransferase that may play a role in the biosynthesis of mammalian O-mannosyl glycans.

Recent studies have shown that O-mannosyl glycans are present in several mammalian glycoproteins. Although knowledge on the functional roles of these glycans is accumulating, their biosynthetic pathways are poorly understood. Here we report the identification and initial characterization of a novel enzyme capable of forming GlcNAc beta 1-2Man linkage, namely UDP-N-acetylglucosamine: O-linked mannose beta-1,2-N-acetylglucosaminyltransferase in the microsome fraction of newborn rat brains. The enzyme transfers GlcNAc to beta-linked mannose residues, and the formed linkage was confirmed to be beta 1-2 on the basis of diplococcal beta-N-acetylhexosaminidase susceptibility and by high-pH anion-exchange chromatography. Its activity is linearly dependent on time, protein concentration, and substrate concentration and is enhanced in the presence of manganese ion. Its activity is not due to UDP-N-acetylglucosamine: alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT-I) or UDP-N-acetylglucosamine: alpha-6-D-mannoside beta-1,2-D-acetylglucosaminyltransferase II (GnT-II), which acts on the early steps of N-glycan biosynthesis, because GnT-I or GnT-II expressed in yeast cells did not show any GlcNAc transfer activity against a synthetic mannosyl peptide. Taken together, the results suggest that the GlcNAc transferase activity described here is relevant to the O-mannosyl glycan pathway in mammals.