Proteolysis with trypsin of mammalian tyrosinase isoforms from B16 mouse melanoma

Melanins and melanogenesis: methods, standards, protocols

Despite considerable advances in the past decade, melanin research still suffers from the lack of universally accepted and shared nomenclature, methodologies, and structural models. This paper stems from the joint efforts of chemists, biochemists, physicists, biologists, and physicians with recognized and consolidated expertise in the field of melanins and melanogenesis, who critically reviewed and experimentally revisited methods, standards, and protocols to provide for the first time a consensus set of recommended procedures to be adopted and shared by researchers involved in pigment cell research. The aim of the paper was to define an unprecedented frame of reference built on cutting-edge knowledge and state-of-the-art methodology, to enable reliable comparison of results among laboratories and new progress in the field based on standardized methods and shared information.

Tyrosinase processing and intracellular trafficking is disrupted in mouse primary melanocytes carrying the underwhite (uw) mutation. A model for oculocutaneous albinism (OCA) type 4

Oculocutaneous albinism (OCA) type 4 is a newly identified human autosomal recessive hypopigmentary disorder that disrupts pigmentation in the skin, hair and eyes. Three other forms of OCA have been previously characterized, each resulting from the aberrant processing and/or sorting of tyrosinase, the enzyme critical to pigment production in mammals. The disruption of tyrosinase trafficking occurs at the level of the endoplasmic reticulum (ER) in OCA1 and OCA3, but at the post-Golgi level in OCA2. The gene responsible for OCA4 is the human homologue of the mouse underwhite (uw) gene, which encodes the membrane-associated transporter protein (MATP). To characterize OCA4, we investigated the processing and sorting of melanogenic proteins in primary melanocytes derived from uw/uw mice and from wild-type mice. OCA4 melanocytes were found to be constantly secreted into the medium dark vesicles that contain tyrosinase and two other melanogenic enzymes, Tyrp1 (tyrosinase-related protein 1) and Dct (DOPAchrome tautomerase); this secretory process is not seen in wild-type melanocytes. Although tyrosinase was synthesized at comparable rates in wild-type and in uw-mutant melanocytes, tyrosinase activity in uw-mutant melanocytes was only about 20% of that found in wild-type melanocytes, and was enriched only about threefold in melanosomes compared with the ninefold enrichment in wild-type melanocytes. OCA4 melanocytes showed a marked difference from wild-type melanocytes in that tyrosinase was abnormally secreted from the cells, a process similar to that seen in OCA2 melanocytes, which results from a mutation of the pink-eyed dilution (P) gene. The P protein and MATP have 12 transmembrane regions and are predicted to function as transporters. Ultrastructural analysis shows that the vesicles secreted from OCA4 melanocytes are mostly early stage melanosomes. Taken together, our results show that in OCA4 melanocytes, tyrosinase processing and intracellular trafficking to the melanosome is disrupted and the enzyme is abnormally secreted from the cells in immature melanosomes, which disrupts the normal maturation process of those organelles. This mechanism explains the hypopigmentary phenotype of these cells and provides new insights into the involvement of transporters in the normal physiology of melanocytes.

Mislocalization of melanosomal proteins in melanocytes from mice with oculocutaneous albinism type 2

More than 10% of admissions worldwide to institutions for the visually impaired are due to some form of albinism. The most common form, oculocutaneous albinism type 2, results from mutations at the p locus. The function of the p gene is yet to be determined. It has been shown that melanocytes from p -null mice exhibit an abnormal melanosomal ultrastructure in addition to alterations in activity and localization of tyrosinase, a critical melanogenic enzyme. In light of these observations, we examined tyrosinase trafficking in p -null vs wildtype mouse melanocytes in order to explore p function. Electron microscopy of wildtype melan-a and p -null melan-p1 cells demonstrated accumulation of tyrosinase in 50 nm vesicles throughout the cell in the absence of p, an observation corroborated by an increase in tyrosinase activity in vesicle-enriched fractions from melan-p1 compared to melan-a cells. Misrouting in the absence of p was not limited to tyrosinase; a second melanosomal protein, tyrosinase-related protein 1, also trafficked incorrectly. In melan-p1, mislocalization led to secretion of tyrosinase into the medium. Adding tyrosine to the medium was found to partially correct tyrosinase trafficking and to reduce secretion; the cysteine protease inhibitor E64 also reduced secretion. We propose that p is required by melanocytes for transport of melanosomal proteins. In its absence, tyrosinase accumulates in vesicles and, in cultured melanocytes, is proteolysed and secreted.

Tyrosinase stabilization by Tyrp1 (the brown locus protein)

Mammalian melanogenesis is regulated directly or indirectly by over 85 distinct loci. The Tyr/albino locus, in which mutations cause a lack of pigmentation, encodes tyrosinase (Tyr), the critical and rate-limiting melanogenic enzyme. Other melanogenic enzymes include Tyrp1 (or TRP1) and 3,4-dihydroxyphenylalanine-chrome tautomerase (Dct or TRP2) encoded at the Tyrp1/brown and Dct/slaty loci, respectively. Murine Tyrp1 can oxidize 5, 6-dihydroxyindole-2-carboxylic acid (DHICA) produced by Dct, but mutations in Tyrp1 also affect the catalytic functions of Tyr. All three enzymes are membrane-bound melanosomal proteins with similar structural features and are thought to interact within and stabilize a melanogenic complex. We have now further investigated the effect of a Tyrp1(b) mutation on Tyr stability. Pulse/chase labeling experiments show that Tyr is degraded more quickly in Tyrp1(b) mutant melanocytes than in melanocytes wild type at that locus. This reduced stability of Tyr can be partly rescued by infection with the wild type Tyrp1 gene, and this is accompanied by phenotypic rescue of infected melanocytes. In sum, these results suggest that, in addition to its catalytic function in oxidizing DHICA, Tyrp1 may play an important role in stabilizing Tyr, a second potential role in the regulation of melanin formation.

Mutational analysis of copper binding by human tyrosinase

Tyrosinase (EC 1.14.18.1) is a copper-containing enzyme that catalyzes several reactions in the biosynthesis of melanin pigments and is deficient in patients with type I oculocutaneous albinism (OCA1). Tyrosinase is thought to bind two copper ions, one at each of two conserved sequence motifs, termed CuA and CuB, but to date this has been directly proved only for the Neurospora and mushroom enzyme. Here, we demonstrate that mammalian tyrosinase directly binds copper, and that the CuA and CuB sites are both required for copper binding and for catalytic activity. We show that in human tyrosinase, copper binding by the CuB site is most likely coordinated by residues His363, His367, and His389, and that copper binding may be cooperative, with copper binding at one site facilitating copper binding by the other site. Furthermore, correct folding of the tyrosinase polypeptide appears to be necessary for copper binding, and a number of human OCA1 mutations disrupt copper binding and thus catalytic function of tyrosinase.

Inhibition of N-glycan processing in B16 melanoma cells results in inactivation of tyrosinase but does not prevent its transport to the melanosome

Tyrosinase is the key enzyme in melanin biosynthesis, catalyzing multiple steps in this pathway. The mature glycoprotein is transported from the Golgi to the melanosome where melanin biosynthesis occurs. In this study, we have investigated the effects of inhibitors of N-glycan processing on the synthesis, transport, and catalytic activity of tyrosinase. When B16 mouse melanoma cells were cultured in the presence of N-butyldeoxynojirimycin, an inhibitor of the endoplasmic reticulum-processing enzymes alpha-glucosidases I and II, the enzyme was synthesized and transported to the melanosome but almost completely lacked catalytic activity. The cells contained only 2% of the melanin found in untreated cells. Structural analysis of the N-glycans from N-butyldeoxynojirimycin-treated B16 cells demonstrated that three oligosaccharide structures (Glc3Man7-9) predominated. Removal of the glucose residues with alpha-glucosidases I and II failed to restore enzymatic activity, suggesting that the glucosylated N-glycans do not sterically interfere with the enzyme's active sites. The mannosidase inhibitor deoxymannojirimycin had no effect on catalytic activity suggesting that the retention of glucosylated N-glycans results in the inactivation of this enzyme. The retention of glucosylated N-glycans does not therefore result in misfolding and degradation of the glycoprotein, as the enzyme is transported to the melanosome, but may cause conformational changes in its catalytic domains.

Biochemical characterization of the melanogenic system in the eye of adult rodents

The melanogenic activities in the eye of the adult gerbil (Meriones unguiculatus) have been investigated and compared to those found in the B16 mouse melanoma model. Eye extracts contain tyrosine hydroxylase, DOPA oxidase, DOPAchrome tautomerase and DHICA oxidase activities. The subcellular distribution of these activities was investigated by differential centrifugation and detergent solubilization of the particulate fractions. The distribution pattern closely resembled the one found for mouse melanoma, with a higher percentage of activity associated to the particulate fractions but a substantial proportion in the cytosolic fraction. The tyrosine hydroxylase activity was characterized by a KM of 62 microM for L-tyrosine and a stringent requirement for the co-factor L-DOPA (Ka 10.3 microM). The KM for L-DOPA was 0.41 mM. The sensitivity of the eye and mouse melanoma tyrosinase activity to a variety of substrate analogs and metal chelators was found to be identical. In keeping with these kinetic similarities, eye tyrosinase displayed some structural properties resembling those of the melanoma enzyme. The molecular weight of the enzyme, determined by SDS-PAGE and DOPA oxidase activity stain, was 75 kDa for the eye enzyme and 66.2 kDa for melanoma tyrosinase, and both enzymes were apparently dimeric in non ionic detergent solution. Immunoprecipitation with specific antibodies proved that at least 80% of the total tyrosinase activity could be immunoprecipitated with the specific anti-tyrosinase antibody alpha PEP7, while the anti-TRP-1 monoclonal antibody TMH-1 precipitated little, if any, tyrosinase activity. Taken together, these observations provide the first vis-à-vis comparison of an extracutaneous melanogenic system and the melanogenic system of melanoma. Our results prove that, at least in rodents, the melanogenic system in the eye is similar, but not identical, to the melanin biosynthesis machinery of epidermal melanocytes.

Effect of detergents and endogenous lipids on the activity and properties of tyrosinase and its related proteins

Within mammalian melanocytes, melanin biosynthesis is controlled by three enzymes structurally related: tyrosinase and two tyrosinase related proteins, TRP1 and TRP2. These melanosomal enzymes are integral membrane proteins with a carboxyl tail oriented to the cytoplasm, a single membrane-spanning helix and the bulk of the protein located inside the melanosome. Their solubilization is usually carried out by treatment of melanosomal preparations with non-ionic detergents, but, so far, no comparative study of the effect of the detergents employed on the properties of the solubilized proteins has been reported. We have compared the effect of the detergents Brij-35, Nonidet P-40, Tween-20, sodium deoxycholate and Triton X-114 on several properties of the melanogenic enzymes, including the solubilization yield, stability, electrophoretic behaviour and accessibility of epitopes located in the carboxyl tail to specific antibodies. Our data indicate that not only the total amount of enzymes solubilized, but also their relative proportions in the solubilized preparations depend on the detergent used. The non-ionic detergents apparently interact strongly with the melanogenic enzymes, affecting their mobility in SDS-PAGE, and might induce different conformations of the carboxyl tail. Complete replacement of lipids by the detergents results in a decreased stability that can be partially reversed by the addition of endogenous lipids. This treatment also produces a noticeable activation of the tyrosinase isoenzymes, which is higher for TRP1 than for tyrosinase. Taken together, these data show that the transmembrane and carboxyl fragments of the proteins of the tyrosinase family might modulate the stability and activity of the melanogenic enzymes.