Cell type-specific storage of dopamine beta-monooxygenase

A membrane-bound dopamine β-hydroxylase highly expressed in granulocyte of Pacific oyster Crassostrea gigas

Dopamine β-hydroxylase (DBH) is one of key rate-limiting enzymes converting dopamine to norepinephrine. It locates not only in catecholaminergic neuron system, but also in immunocytes and plays roles in the immune response of vertebrates. However, the knowledge about the function of DBH in immune system is still very limited in invertebrates. In the present study, the DBH gene family with seven members was screened from Crassostrea gigas genome, and their mRNA expressions in various tissues were recorded. Among them, one DBH (designated CgDBH-1) with high expression level in oyster hemocytes was further characterized. The deduced amino acid sequence of CgDBH-1 was predicted to contain a transmembrane domain and shared 30.1% and 30.9% similarity with that in Mus musculus and Homo sapiens, respectively. CgDBH-1 was closely clustered with DBH from Aplysia californica in the phylogenetic tree. The recombinant protein of CgDBH-1 (rCgDBH-1) exhibited significant enzymatic activity (0.54 ± 0.019 pmol L^-1 min^-1) to synthesize norepinephrine. Importantly, the mRNA transcript of CgDBH-1 was highly expressed in oyster hemocytes, and the highest expression level was observed in granulocytes among the three types of hemocytes, which was 8.18-fold (p < 0.01) of that in agranulocytes. Moreover, the expression of CgDBH-1 in hemocytes was significantly increased at the late stage of immune response. The CgDBH-1 protein was mainly co-localized with the granules and endoplasmic reticulum (ER) of granulocytes. These results collectively suggested that CgDBH-1, as a novel molluscan norepinephrine synthesizing enzyme highly expressed in granulocytes, involved in the late-stage immune response of oysters, which provided vital insight to understand the crosstalk between neuroendocrine and immune systems in invertebrates.

Characterization of the peptidylglycine α-amidating monooxygenase (PAM) from the venom ducts of neogastropods, Conus bullatus and Conus geographus

Cone snails, genus Conus, are predatory marine snails that use venom to capture their prey. This venom contains a diverse array of peptide toxins, known as conotoxins, which undergo a diverse set of posttranslational modifications. Amidating enzymes modify peptides and proteins containing a C-terminal glycine residue, resulting in loss of the glycine residue and amidation of the preceding residue. A significant fraction of peptides present in the venom of cone snails contain C-terminal amidated residues, which are important for optimizing biological activity. This study describes the characterization of the amidating enzyme, peptidylglycine α-amidating monooxygenase (PAM), present in the venom duct of cone snails, Conus bullatus and Conus geographus. PAM is known to carry out two functions, peptidyl α-hydroxylating monooxygenase (PHM) and peptidylamido-glycolate lyase (PAL). In some animals, such as Drosophila melanogaster, these two functions are present in separate polypeptides, working as individual enzymes. In other animals, such as mammals and in Aplysia californica, PAM activity resides in a single, bifunctional polypeptide. Using specific oligonucleotide primers and reverse transcription-polymerase chain reaction we have identified and cloned from the venom duct cDNA library, a cDNA with 49% homology to PAM from A. californica. We have determined that both the PHM and PAL activities are encoded in one mRNA polynucleotide in both C. bullatus and C. geographus. We have directly demonstrated enzymatic activity catalyzing the conversion of dansyl-YVG-COOH to dansyl-YV-NH2 in cloned cDNA expressed in Drosophila S2 cells.

Norepinephrine deficiency is caused by combined abnormal mRNA processing and defective protein trafficking of dopamine beta-hydroxylase

Human norepinephrine (NE) deficiency (or dopamine β-hydroxylase (DBH) deficiency) is a rare congenital disorder of primary autonomic failure, in which neurotransmitters NE and epinephrine are undetectable. Although potential pathogenic mutations, such as a common splice donor site mutation (IVS1+2T→C) and various missense mutations, in NE deficiency patients were identified, molecular mechanisms underlying this disease remain unknown. Here, we show that the IVS1+2T→C mutation results in a non-detectable level of DBH protein production and that all three missense mutations tested lead to the DBH protein being trapped in the endoplasmic reticulum (ER). Supporting the view that mutant DBH induces an ER stress response, exogenous expression of mutant DBH dramatically induced expression of BiP, a master ER chaperone. Furthermore, we found that a pharmacological chaperone, glycerol, significantly rescued defective trafficking of mutant DBH proteins. Taken together, we propose that NE deficiency is caused by the combined abnormal processing of DBH mRNA and defective protein trafficking and that this disease could be treated by a pharmacological chaperone(s).

Monooxygenase X, a member of the copper-dependent monooxygenase family localized to the endoplasmic reticulum

Based on sequence comparisons, MOX (monooxygenase X), is a member of the copper monooxygenase family that includes dopamine beta-monooxygenase (DBM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM). MOX has all of the residues expected to be critical for copper binding, and its cysteine residues can yield the intramolecular disulfide bond pattern observed in DBM. Although DBM and PHM function within the lumen of the secretory pathway, the published sequence for human MOX lacks a signal sequence, suggesting that it does not enter this compartment. We identified an upstream exon that encodes the signal sequence of human MOX. A retained intron yields minor amounts of transcript encoding MOX without a signal sequence. MOX transcripts are widely expressed, with the highest levels in the salivary gland and ovary and moderate levels in brain, pituitary, and heart. Despite the presence of a signal sequence, exogenous MOX is not secreted, and it localizes throughout the endoplasmic reticulum in both endocrine or nonendocrine cells. Neither appending green fluorescent protein to its C terminus nor deleting the hydrophobic domain near its C terminus facilitates secretion of MOX. MOX is N-glycosylated, is tightly membrane-associated, and forms oligomers that are not disulfide-linked. Based on its sequence and localization, MOX is predicted to hydroxylate a hydrophobic substrate in the endoplasmic reticulum.

A functionally atypical amidating enzyme from the human parasite Schistosoma mansoni

Many neuropeptide transmitters require the presence of a carboxy-terminal alpha-amide group for biological activity. Amidation requires conversion of a glycine-extended peptide intermediate into a C-terminally amidated product. This post-translational modification depends on the sequential action of two enzymes (peptidylglycine alpha-hydroxylating monooxygenase or PHM, and peptidyl-alpha-hydroxyglycine alpha-amidating lyase or PAL) that in most eukaryotes are expressed as separate domains of a single protein (peptidylglycine alpha-amidating monooxygenase or PAM). We identified a cDNA encoding PHM in the human parasite Schistosoma mansoni. Transient expression of schistosome PHM (smPHM) revealed functional properties that are different from other PHM proteins; smPHM displays a lower pH-optimum and, when expressed in mammalian cells, is heavily N-glycosylated. In adult worms, PHM is found in the trans-Golgi network and secretory vesicles of both central and peripheral nerves. The widespread occurrence of PHM in the nervous system confirms the important role of amidated neuropeptides in these parasitic flatworms. The differences between schistosome and mammalian PHM suggest that it could be a target for new chemotherapeutics.

Dopamine beta-monooxygenase signal/anchor sequence alters trafficking of peptidylglycine alpha-hydroxylating monooxygenase

Dopamine beta-monooxygenase (DBM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM) are essential for the biosynthesis of catecholamines and amidated peptides, respectively. The enzymes share a conserved catalytic core. We studied the role of the DBM signal sequence by appending it to soluble PHM (PHMs) and expressing the DBMsignal/PHMs chimera in AtT-20 and Chinese hamster ovary cells. PHMs produced as part of DBMsignal/PHMs was active. In vitro translated and cellular DBMsignal/PHMs had similar masses, indicating that the DBM signal was not removed. DBMsignal/PHMs was membrane-associated and had the properties of an intrinsic membrane protein. After in vitro translation in the presence of microsomal membranes, trypsin treatment removed 2 kDa from DBMsignal/PHMs while PHMs was entirely protected. In addition, a Cys residue in DBMsignal/PHMs was accessible to Cys-directed biotinylation. Thus the chimera adopts the topology of a type II membrane protein. Pulse-chase experiments indicate that DBMsignal/PHMs turns over rapidly after exiting the trans-Golgi network. Although PHMs is efficiently localized to secretory granules, DBMsignal/PHMs is largely localized to the endoplasmic reticulum in AtT-20 cells. On the basis of stimulated secretion, the small amount of PHMs generated is stored in secretory granules. In contrast, the expression of DBMsignal/PHMs in PC12 cells yields protein that is localized to secretory granules.

DBHR, a gene with homology to dopamine beta-hydroxylase, is expressed in the neural crest throughout early development

In a screen for genes involved in neural crest development, we identified DBHR (DBH-Related), a putative monooxygenase with low homology to dopamine beta-hydroxylase (DBH). Here, we describe novel expression patterns for DBHR in the developing embryo and particularly the neural crest. DBHR is an early marker for prospective neural crest, with earliest expression at the neural plate border where neural crest is induced. Furthermore, DBHR expression persists in migrating neural crest and in many, though not all, crest derivatives. DBHR is also expressed in the myotome, from the earliest stages of its formation, and in distinct regions of the neural tube, including even-numbered rhombomeres of the hindbrain. In order to investigate the signals that regulate its segmented pattern in the hindbrain, we microsurgically rotated the rostrocaudal positions of rhombomeres 3/4. Despite their ectopic position, both rhombomeres continued to express DBHR at the level appropriate for their original location, indicating that DBHR is regulated autonomously within rhombomeres. We conclude that DBHR is a divergent member of a growing family of DBH-related genes; thus, DBHR represents a completely new type of neural crest marker, expressed throughout the development of the neural crest, with possible functions in cell-cell signaling.

Cell type-dependent differences in thyroid peroxidase cell surface expression

Recently, it has been suggested that only approximately 2% of human thyroid peroxidase (hTPO(933)) reaches the surface of stably transfected (Chinese hamster ovary) cells, most being degraded intracellularly, and this might be representative of thyroid peroxidase (TPO) behavior in thyrocytes (Fayadat, L., Siffroi-Fernandez, S., Lanet, J., and Franc, J.-L. (2000) J. Biol. Chem. 275, 15948-15954). In agreement, in stably transfected Madin-Darby canine kidney clones, nonpermeabilized cells exhibit wild-type hTPO(933) immunofluorescence (apically) on <10% of that found in permeabilized cells, where an endoplasmic reticulum pattern is observed. Further, a C-terminally truncated, membrane-anchorless hTPO(848) is also retained in the endoplasmic reticulum of stably transfected Madin-Darby canine kidney cells. However, by contrast, in Chinese hamster ovary cells after transient transfection, hTPO(933) immunofluorescence is detected equally well in nonpermeabilized and permeabilized cells, indicating that a large portion of hTPO(933) is present at the cell surface; furthermore, hTPO(848) is efficiently secreted. Further, using an antiserum not cross-reacting with rat TPO, we find by immunofluorescence that in stable clones of PC Cl3 (rat) thyrocytes, considerably more ( approximately 50%) of the cells exhibit hTPO(933) at the cell surface. However, cell surface biotinylation and endoglycosidase H digestion assays appear to under-represent the extent of hTPO(933) transport, presumably because protein folding limits both Golgi carbohydrate modification and accessibility of lysines in the extracellular domain. We conclude that cell type-specific factors may facilitate stable expression of TPO at the cell surface of thyrocytes.