Intracellular sorting and targeting of melanosomal membrane proteins: identification of signals for sorting of the human brown locus protein, gp75

Di-leucine signals mediate targeting of tyrosinase and synaptotagmin to synaptic-like microvesicles within PC12 cells

One pathway in forming synaptic-like microvesicles (SLMV) involves direct budding from the plasma membrane, requires adaptor protein 2 (AP2) and is brefeldin A (BFA) resistant. A second route leads from the plasma membrane to an endosomal intermediate from which SLMV bud in a BFA-sensitive, AP3-dependent manner. Because AP3 has been shown to bind to a di-leucine targeting signal in vitro, we have investigated whether this major class of targeting signals is capable of directing protein traffic to SLMV in vivo. We have found that a di-leucine signal within the cytoplasmic tail of human tyrosinase is responsible for the majority of the targeting of HRP-tyrosinase chimeras to SLMV in PC12 cells. Furthermore, we have discovered that a Met-Leu di-hydrophobic motif within the extreme C terminus of synaptotagmin I supports 20% of the SLMV targeting of a CD4-synaptotagmin chimera. All of the traffic to the SLMV mediated by either di-Leu or Met-Leu is BFA sensitive, strongly suggesting a role for AP3 and possibly for an endosomal intermediate in this process. The differential reduction in SLMV targeting for HRP-tyrosinase and CD4-synaptotagmin chimeras by di-alanine substitutions or BFA treatment implies that different proteins use the two routes to the SLMV to differing extents.

Protein kinase C-beta activates tyrosinase by phosphorylating serine residues in its cytoplasmic domain

We have previously shown that protein kinase C-beta (PKC-beta) is required for activation of tyrosinase (Park, H. Y., Russakovsky, V., Ohno, S., and Gilchrest, B. A. (1993) J. Biol. Chem. 268, 11742-11749), the rate-limiting enzyme in melanogenesis. We now examine its mechanism of activation in human melanocytes. In vivo phosphorylation experiments revealed that tyrosinase is phosphorylated through the PKC-dependent pathway and that introduction of PKC-beta into nonpigmented human melanoma cells lacking PKC-beta lead to the phosphorylation and activation of tyrosinase. Preincubation of intact melanosomes with purified active PKC-beta in vitro increased tyrosinase activity 3-fold. By immunoelectron microscopy, PKC-beta but not PKC-alpha was closely associated with tyrosinase on the outer surface of melanosomes. Western blot analysis confirmed the association of PKC-beta with melanosomes. Only the cytoplasmic (extra-melanosomal) domain of tyrosinase, which contains two serines but no threonines, was phosphorylated by the serine/threonine kinase PKC-beta. These two serines at positions 505 and 509 both are present in the C-terminal peptide generated by trypsin digestion of tyrosinase. Co-migration experiments comparing synthetic peptide standards of all three possible phosphorylated tryptic peptides, a diphosphopeptide and two monophosphopeptides, to tyrosinase-phosphorylated in intact melanocytes by PKC-beta and then subjected to trypsin digestion revealed that both serine residues are phosphorylated by PKC-beta. We conclude that PKC-beta activates tyrosinase directly by phosphorylating serine residues at positions 505 and 509 in the cytoplasmic domain of this melanosome-associated protein.

A cytoplasmic sequence in human tyrosinase defines a second class of di-leucine-based sorting signals for late endosomal and lysosomal delivery

Distinct cytoplasmic sorting signals target integral membrane proteins to late endosomal compartments, but it is not known whether different signals direct targeting by different pathways. The availability of multiple pathways may permit some cell types to divert proteins to specialized compartments, such as the melanosome of pigmented cells. To address this issue, we characterized sorting determinants of tyrosinase, a tissue-specific resident protein of the melanosome. The cytoplasmic domain of tyrosinase was both necessary and sufficient for internalization and steady state localization to late endosomes and lysosomes in HeLa cells. Mutagenesis of two leucine residues within a conventional di-leucine motif ablated late endosomal localization. However, the properties of this di-leucine-based signal were distinguished from that of CD3gamma by overexpression studies; overexpression of the tyrosinase signal, but not the well characterized CD3gamma signal, induced a 4-fold enlargement of late endosomes and lysosomes and interfered with endosomal sorting mediated by both tyrosine- and other di-leucine-based signals. These properties suggest that the tyrosinase and CD3gamma di-leucine signals are distinctly recognized and sorted by distinct pathways to late endosomes in non-pigmented cells. We speculate that melanocytic cells utilize the second pathway to divert proteins to the melanosome.

Identification of MAGE-3 epitopes presented by HLA-DR molecules to CD4(+) T lymphocytes

MAGE-type genes are expressed by many tumors of different histological types and not by normal cells, except for male germline cells, which do not express major histocompatibility complex (MHC) molecules. Therefore, the antigens encoded by MAGE-type genes are strictly tumor specific and common to many tumors. We describe here the identification of the first MAGE-encoded epitopes presented by histocompatibility leukocyte antigen (HLA) class II molecules to CD4(+) T lymphocytes. Monocyte-derived dendritic cells were loaded with a MAGE-3 recombinant protein and used to stimulate autologous CD4(+) T cells. We isolated CD4(+) T cell clones that recognized two different MAGE-3 epitopes, MAGE-3114-127 and MAGE-3121-134, both presented by the HLA-DR13 molecule, which is expressed in 20% of Caucasians. The second epitope is also encoded by MAGE-1, -2, and -6. Our procedure should be applicable to other proteins for the identification of new tumor-specific antigens presented by HLA class II molecules. The knowledge of such antigens will be useful for evaluation of the immune response of cancer patients immunized with proteins or with recombinant viruses carrying entire genes coding for tumor antigens. The use of antigenic peptides presented by class II in addition to peptides presented by class I may also improve the efficacy of therapeutic antitumor vaccination.

The tyrosinase tail mediates sorting to the lysosomal compartment in MDCK cells via a di-leucine and a tyrosine-based signal

Tyrosinase is a type I membrane protein found in melanosomes, which are lysosomal-like organelles and specific for pigment cells. A mutation of mouse tyrosinase, platinum (cp), leads to truncation of tyrosinase's cytosolic tail, and results in misrouting to the cell periphery. In this study, we expressed chimeras of wild-type and mutant cytosolic tails of mouse tyrosinase fused to rat lysosome-associated membrane protein-1 luminal and transmembrane domain to study sorting of tyrosinase in Madin-Darby canine kidney cells. The study shows that the mouse tyrosinase cytosolic tail is necessary and sufficient to mediate sorting of a heterologous type I membrane protein to compartments of the lysosomal lineage. Whereas deletions of 7 or 10 C-terminal amino acids of the tail still result in sorting to lysosomes, a deletion mutant corresponding to platinum (cp) tail fails to sort correctly and corroborates the in situ findings in cp homozygous mutant mice. Correct sorting of tyrosinase-lysosome-associated membrane protein-1 chimeras is mediated by the interplay of a di-leucine signal and a tyrosine motif of the Y-X-X-O type.

Altered trafficking of lysosomal proteins in Hermansky-Pudlak syndrome due to mutations in the beta 3A subunit of the AP-3 adaptor

Hermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by defective lysosome-related organelles. Here, we report the identification of two HPS patients with mutations in the beta 3A subunit of the heterotetrameric AP-3 complex. The patients' fibroblasts exhibit drastically reduced levels of AP-3 due to enhanced degradation of mutant beta 3A. The AP-3 deficiency results in increased surface expression of the lysosomal membrane proteins CD63, lamp-1, and lamp-2, but not of nonlysosomal proteins. These differential effects are consistent with the preferential interaction of the AP-3 mu 3A subunit with tyrosine-based signals involved in lysosomal targeting. Our results suggest that AP-3 functions in protein sorting to lysosomes and provide an example of a human disease in which altered trafficking of integral membrane proteins is due to mutations in a component of the sorting machinery.

Normal tyrosine transport and abnormal tyrosinase routing in pink-eyed dilution melanocytes

The pink-eyed dilution phenotype in mice arises from mutations in the p gene; in humans, analogous mutations in the P gene result in oculocutaneous albinism type 2. Although the molecular mechanisms which underlie this phenotype remain obscure, it has been postulated that mutations in p result in defective tyrosine transport into murine melanosomes, resulting in hypopigmentation and diminished coat color. However, we previously reported no difference in melanosomal tyrosine transport in unpigmented, melanoblast-like pink-eyed dilution (pcp/pcp), and in pigmented (melan-a) murine melanocytes. In this study, we utilized melan-p1 cells, more differentiated pink-eyed dilution (pcp/p25H) melanocytes which can be induced to produce melanin, to characterize the melanogenic lesion(s) more definitively. Uptake of [3H]tyrosine into melan-a melanosomes did not differ significantly from uptake into melanosomes derived from melan-p1 melanocytes, further arguing against its critical role as a tyrosine transporter. Pink-eyed dilution melanocytes incubated in high tyrosine concentrations became extremely pigmented as they became confluent and secreted large amounts of black material into the medium. Total cellular tyrosinase activity in melan-p1 melanocytes was significantly higher than that in melan-a melanocytes (which are wild-type at the p locus), but the localization of tyrosinase to melanosomes was impaired in melan-p1 melanocytes compared to melan-a melanocytes. These results indicate that mechanisms other than deficient tyrosine transport are involved in the pink-eyed dilution phenotype and that this protein may serve a chaperone-like or stabilizing function in melanocytes.

Tumor immunity and autoimmunity induced by immunization with homologous DNA

The immune system can recognize self antigens expressed by cancer cells. Differentiation antigens are prototypes of these self antigens, being expressed by cancer cells and their normal cell counterparts. The tyrosinase family proteins are well characterized differentiation antigens recognized by antibodies and T cells of patients with melanoma. However, immune tolerance may prevent immunity directed against these antigens. Immunity to the brown locus protein, gp75/ tyrosinase-related protein-1, was investigated in a syngeneic mouse model. C57BL/6 mice, which are tolerant to gp75, generated autoantibodies against gp75 after immunization with DNA encoding human gp75 but not syngeneic mouse gp75. Priming with human gp75 DNA broke tolerance to mouse gp75. Immunity against mouse gp75 provided significant tumor protection. Manifestations of autoimmunity were observed, characterized by coat depigmentation. Rejection of tumor challenge required CD4(+) and NK1.1(+) cells and Fc receptor gamma-chain, but depigmentation did not require these components. Thus, immunization with homologous DNA broke tolerance against mouse gp75, possibly by providing help from CD4(+) T cells. Mechanisms required for tumor protection were not necessary for autoimmunity, demonstrating that tumor immunity can be uncoupled from autoimmune manifestations.

Endocytosis of the common cytokine receptor gammac chain. Identification of sequences involved in internalization and degradation

The common cytokine receptor gammac, shared by interleukin 2, 4, 7, 9, and 15 receptors, has a major role in lymphocyte proliferation and differentiation, leading, when mutated, to a genetic disease, X-linked severe combined immunodeficiency. In this study, we report that gammac is internalized and degraded in lymphoid cells. To identify gammac regions involved in sorting along the endocytic pathway, we have studied a chimeric protein composed of the extracellular part of interleukin 2-receptor alpha and transmembrane and intracellular part of gammac, alpha gamma gammawt. When transfected in Jurkat T cells, alpha gamma gammawt is as efficiently internalized and degraded as gammac, demonstrating that the transmembrane and cytosolic tail of gammac carry sequences involved in this process. To identify these motifs, we have analyzed the trafficking of chimeric proteins with serial truncations in their cytosolic tail. Internalization studies showed that the cytosolic tail of gammac contains three regions located between cytosolic amino acids 1-35, 35-40, and 40-65 involved in gammac endocytosis. Successive deletions of these motifs result in reduced endocytosis. One region containing the 5 cytosolic amino acids 36-40 is essential to direct gammac to the degradation pathway. These sorting sequences, by participating in the fine tuning of cell surface gammac expression, might somewhat regulate the cell responsiveness to interleukins whose receptors share this component.

A di-leucine-based motif in the cytoplasmic tail of LIMP-II and tyrosinase mediates selective binding of AP-3

Among the various coats involved in vesicular transport, the clathrin associated coats that contain the adaptor complexes AP-1 and AP-2 are the most extensively characterized. The function of the recently described adaptor complex AP-3, which is similar to AP-1 and AP-2 in protein composition but does not associate with clathrin, is not known. By monitoring surface plasmon resonance we observed that AP-3 is able to interact with the tail of the lysosomal integral membrane protein LIMP-II and that this binding depends on a DEXXXLI sequence in the LIMP-II tail. Furthermore, AP-3 bound to the cytoplasmic tail of the melanosome-associated protein tyrosinase which contains a related EEXXXLL sequence. The tails of LIMP-II and tyrosinase either did not interact, or only interacted poorly, with AP-1 or AP-2. In contrast, the cytoplasmic tails of other membrane proteins containing di-leucine and/or tyrosine-based sorting signals did not bind AP-3, but AP-1 and/or AP-2. This points to a function of AP-3 in intracellular sorting to lysosomes and melanosomes of a subset of cargo proteins via di-leucine-based sorting motifs.

What controls melanogenesis?

The pigments eumelanin and pheomelanin are the visually most striking products of specialized neural crest-derived cells (melanocytes), and provide color to both epidermis and hair shafts. While the intriguing and controversial biological functions of these multifaceted heteropolymers will be discussed in a later feature, here it is explored how their generation (melanogenesis) is controlled. For decades, this has been the object of much controversy, the salient features of which are delineated in the following contributions.

Human tyrosinase related protein-1 (TRP-1) does not function as a DHICA oxidase activity in contrast to murine TRP-1

Tyrosinase related protein-1 is a melanocyte specific protein and a member of the tyrosinase gene family which also includes tyrosinase and TRP 2 (DOPAchrome tautomerase). In murine melanocytes, TRP-1 functions as a 5,6-dihydroxyindole-2-carboxylic acid [DHICA] oxidase during the biosynthetic conversion of tyrosine to eumelanin and mutations affecting TRP-1 result in the synthesis of brown rather than black pelage coloration. In this study, we examined the putative DHICA oxidase activity of TRP-1 in human melanocytes using several approaches. We first utilized a line of cultured melanocytes established from a patient with a form of oculocutaneous albinism completely lacking expression of TRP-1 (OCA3). This line of melanocytes endogenously exhibited the same amount of DHICA oxidase activity as control melanocytes expressing TRP-1. In other experiments, cultured human fibroblasts were transfected with a cDNA for TRP-1, in either the sense or antisense direction, or with the retroviral vector alone. TRP-1 expression was induced in fibroblasts transfected with the TRP-1 cDNA in the sense direction only. Although TRP-1 was expressed by sense-transfected cells, there was no significant DHICA oxidase activity above controls. These results demonstrate that human TRP-1 does not use DHICA as a substrate for oxidation.

The AP-3 complex: a coat of many colours

A new adaptor protein complex, termed AP-3, has recently been identified in mammalian cells, and genetic studies in yeast have revealed a functional role for the AP-3 complex in cargo-selective transport via a new alternative trafficking pathway from the Golgi to the vacuole/lysosome. Here, the authors review what is currently known about the AP-3 complex and discuss recent insight into its function in multicellular organisms that has come from the finding that mutations in AP-3 subunits correspond to classical mutations in Drosophila and mice.

An atypical sorting determinant in the cytoplasmic domain of P-selectin mediates endosomal sorting

We previously identified the 11 amino acid C1 region of the cytoplasmic domain of P-selectin as essential for an endosomal sorting event that confers rapid turnover on P-selectin. The amino acid sequence of this region has no obvious similarity to other known sorting motifs. We have analyzed the sequence requirements for endosomal sorting by measuring the effects of site-specific mutations on the turnover of P-selectin and of the chimeric protein LLP, containing the lumenal and transmembrane domains of the low density lipoprotein receptor and the cytoplasmic domain of P-selectin. Endosomal sorting activity was remarkably tolerant of alanine substitutions within the C1 region. The activity was eliminated by alanine substitution of only one amino acid residue, leucine 768, where substitution with several other large side chains, hydrophobic and polar, maintained the sorting activity. The results indicate that the endosomal sorting determinant is not structurally related to previously reported sorting determinants. Rather, the results suggest that the structure of the sorting determinant is dependent on the tertiary structure of the cytoplasmic domain.

The cytoplasmic tail of the mouse brown locus product determines intracellular stability and export from the endoplasmic reticulum

Several melanosome membrane proteins have been identified, forming a family of proteins known as tyrosinase related proteins. Human TRP-1/gp75 is sorted to melanosomes through the endoplasmic reticulum and Golgi complex to the endocytic pathway, directed by a sorting signal located in the cytoplasmic tail. This hexapeptide cytoplasmic sequence, which is conserved in the tyrosinase related protein family and through vertebrate evolution, was shown to act also as a sorting signal in mouse gp75, confirming that its sorting and cellular retention function is conserved between human and mouse. The cytoplasmic tail influenced the rate and efficiency of intracellular transport of gp75 from the endoplasmic reticulum to the cis-Golgi. Deletion of 33 or 27 amino acids from the carboxyl end of the 38 amino acid cytoplasmic tail of gp75 caused retention and rapid degradation of the truncated gp75 in the endoplasmic reticulum. This defective movement could be fully corrected by extending the truncated tail with the unrelated cytoplasmic tail of the low density lipoprotein receptor. Thus, the cytoplasmic tail of mouse gp75 not only determines sorting to the endocytic/melanosomal compartment, but also controls export from the endoplasmic reticulum to Golgi.

Protein sorting within the MHC class II antigen-processing pathway

Major histocompatibility complex (MHC) class II molecules are required for the presentation of antigenic peptides that are derived predominantly from internalized proteins. The assembly of MHC class II/peptide complexes occurs within endosomal compartments of antigen-presenting cells (APCs). Therefore, for assembly to occur, MHC class II molecules, foreign proteins, and accessory molecules must be sorted to appropriate intracellular sites. My laboratory is trying to understand how proteins are sorted to various antigen-processing compartments as well as to conventional endosomal organelles. Using chimeric marker proteins and a variety of biochemical and genetic approaches, we are addressing the specificity of protein sorting and the mechanisms by which sorting signals are deciphered. By using a similar chimeric protein approach to target endogenous proteins to distinct compartments, we hope to address the role of processing events in each compartment in the generation of MHC class II ligands.

Autologous Human Monocyte-Derived Dendritic Cells Genetically Modified to Express Melanoma Antigens Elicit Primary Cytotoxic T Cell Responses In Vitro: Enhancement by Cotransfection of Genes Encoding the Th1-Biasing Cytokines IL-12 and IFN-α

DNA-based immunization strategies designed to elicit cellular antitumor immunity offer an attractive alternative to protein- or peptide-based approaches. In the present study we have evaluated the feasibility of DNA vaccination for the induction of CTL reactivity to five different melanoma Ags in vitro. Cultured, monocyte-derived dendritic cells (DC) were transiently transfected with plasmid DNA encoding human MART-1/Melan-A, pMel-17/gp100, tyrosinase, MAGE-1, or MAGE-3 by particle bombardment and used to stimulate autologous PBMC responder T cells. CTL reactivity to these previously identified melanoma Ags was reproducibly generated after two or three stimulations with genetically modified DC. Co-ordinate transfection of two melanoma Ag cDNAs into DC promoted CTL responders capable of recognizing epitopes from both gene products. Coinsertion of genes encoding the Th1-biasing cytokines IL-12 or IFN-α consistently enhanced the magnitude of the resulting Ag-specific CTL reactivity. Importantly, DC transfected with a single melanoma Ag cDNA were capable of stimulating Ag-specific CTL reactivity restricted by multiple host MHC alleles, some of which had not been previously identified. These results support the inherent strengths of gene-based vaccine approaches that do not require prior knowledge of responder MHC haplotypes or of relevant MHC-restricted peptide epitopes. Given previous observations of in situ tumor HLA allele-loss variants, DC gene vaccine strategies may elicit a greater diversity of host therapeutic immunity, thereby enhancing the clinical utility and success of such approaches.

Sorting and secretion of a melanosome membrane protein, gp75/TRP1

The melanosome is an organelle specialized for melanin synthesis that is derived from the endocytic pathway. Several melanosome membrane proteins have been identified, forming a family of proteins known as tyrosinase-related proteins. Two members of this family, tyrosinase and gp75, are well-characterized melanocyte differentiation antigens. Our previous studies have shown that gp75, the mouse brown locus protein, is sorted to melanosomes along the endocytic pathway, directed by a hexapeptide sorting signal located in the cytoplasmic tail. In this study, we report the unexpected finding that a portion of gp75 is secreted. Substantial levels of secretory gp75 were detected in melanocytic cells. Cell surface expression of gp75 was also detected, representing 2% of cellular gp75. Characterization of secretory gp75 cells showed that it is: (i) a truncated form that lacks the transmembrane region, the cytoplasmic tail where the endosomal sorting signal is located, and a small portion of the lumenal domain; (ii) more extensively glycosylated than endocytic/melanosomal gp75, containing trans-Golgi processed sugar residues; and (iii) generated post-translationally in an acid sensitive compartment after processing in the trans-Golgi, and secreted rapidly after generation. Thus, these endocytic/melanosomal membrane proteins can be processed to abundant secretory forms, probably in an endocytic compartment through a potentially novel secretory pathway.

Dendritic cells retrovirally transduced with a model antigen gene are therapeutically effective against established pulmonary metastases

Dendritic cells (DCs) are bone marrow-derived leukocytes that function as potent antigen presenting cells capable of initiating T cell-dependent responses from quiescent lymphocytes. DC pulsed with tumor-associated antigen (TAA) peptide or protein have recently been demonstrated to elicit antigen-specific protective antitumor immunity in a number of murine models. Transduction of DCs with TAA genes may allow stable, prolonged antigen expression as well as the potential for presentation of multiple, or unidentified, epitopes in association with major histocompatibility complex class I and/or class II molecules. To evaluate the potential efficacy of retrovirally transduced DCs, bone marrow cells harvested from BALB/c mice were transduced with either a model antigen gene encoding beta-galactosidase (beta-gal) or a control gene encoding rat HER-2/neu (Neu) by coculture with irradiated ecotropic retroviral producer lines. Bone marrow cells were differentiated into DC in vitro using granulocyte/macrophage colony-stimulating factor and interleukin-4. After 7 d in culture, cells were 45-78% double positive for DC phenotypic cell surface markers by FACS(R) analysis, and DC transduced with beta-gal were 41-72% positive for beta-gal expression by X-gal staining. In addition, coculture of beta-gal transduced DC with a beta-gal-specific T cell line (CTLx) resulted in the production of large amounts of interferon-gamma, demonstrating that transduced DCs could process and present endogenously expressed beta-gal. DC transduced with beta-gal and control rat HER-2/neu were then used to treat 3-d lung metastases in mice bearing an experimental murine tumor CT26.CL25, expressing the model antigen, beta-gal. Treatment with beta-gal-transduced DC significantly reduced the number of pulmonary metastatic nodules compared with treatment with Hank's balanced salt solution or DCs transduced with rat HER-2/neu. In addition, immunization with beta-gal-transduced DCs resulted in the generation of antigen-specific cytotoxic T lymphocytes (CTLs), which were significantly more reactive against relevant tumor targets than CTLs generated from mice immunized with DCs pulsed with the Ld-restricted beta-gal peptide. The results observed in this rapidly lethal tumor model suggest that DCs transduced with TAA may be a useful treatment modality in tumor immunotherapy.

The predominant defect in dilute melanocytes is in melanosome distribution and not cell shape, supporting a role for myosin V in melanosome transport

Mice with mutations at the dilute locus, which encodes the heavy chain of a type V unconventional myosin, exhibit a reduction in coat colour intensity. This defect is thought to be caused by the absence in dilute melanocytes of the extensive dendritic arbor through which these cells normally deliver pigment-laden melanosomes to keratinocytes. The data on which this conclusion has been based can also be explained, however, by a defect in the outward transport of melanosomes within melanocytes of normal shape. To resolve this question, we compared the shape and pigment distribution within melanocytes present in primary cultures prepared from the epidermis of C57BL/6J pups that were either wild type (D/D) at dilute or homozygous for the dilute null allele d120J. These same comparisons were also performed on melanocytes in situ, where antibodies to the membrane tyrosine kinase receptor cKIT were used to visualize melanocyte cell shape independent of pigment distribution. Wild type melanocytes were found to be dendritic and to have melanosomes distributed throughout their dendrites both in vitro and in situ. Mutant melanocytes were also found to be dendritic in both cases, but their melanosomes were highly concentrated in the cell body and largely excluded from dendrites. We conclude, therefore, that the predominant defect in dilute melanocytes is in melanosome distribution, not cell shape. These results argue that the myosin V isoform encoded by the dilute locus functions in dendritic extensions to move melanosomes from their site of formation within the cell body to their site of intercellular transfer at dendritic tips. This conclusion is consistent with our recent demonstration by immunolocalization that the dilute myosin V isoform associates with melanosomes in mouse melanocytes.

Biological role of tyrosinase related protein and its biosynthesis and transport from TGN to stage I melanosome, late endosome, through gene transfection study

Tyrosinase-related protein (TRP)-1 is one of the most abundant melanosomal glycoproteins involved in melanogenesis. This report summarizes our recent research efforts related to the biological role and biosynthesis of TRP-1 and its transport from TGN (trans-Golgi network) to the stage I melanosome. Our UV irradiation and tyrosinase and TRP-1 cDNA co-transfection studies indicated that human TRP-1 is involved in not only melanogenesis but also prevention of melanocyte death, which may occur during biosynthesis of melanin pigment in the presence of tyrosinase. Furthermore, a coordinated gene interaction was indicated between tyrosinase and TRP-1, resulting in upregulation of mRNA and protein expression of LAMP (lysosome-associated membrane protein)-1 that would directly prevent the tyrosinase-mediated programmed cell death of melanocytes. Similar to tyrosinase, however, TRP-1 appears to require a molecular chaperone, calnexin, which we have cloned recently. Our cDNA transfection study of tyrosinase with calnexin showed clearly the necessity of calnexin in order to have efficient, functional activity of melanosomal glycoprotein, especially tyrosinase. Once glycosylation is completed, TRP-1 will be transported from TGN to the stage I melanosome. At this stage, TRP-1 will have its own target signal, in particular, tyrosine-rich leucine residues in cytoplasmic tail. Our TRP-1 cDNA transfection and immunoelectron microscopy study shows that TRP-1 will be transported through small vesicles, probably non-clathrin-coated type, to large vacuoles, identical to the MPR (mannose-6-phosphate receptor)-positive, late endosomes. In this transport process a low molecular weight G-protein, rab-7, was isolated from the purified melanosomal protein on 2D-PAGE and identified by subsequent sequencing and PCR amplification. Confocal microscopy with double immunostaining and immunoelectron microscopy confirmed the co-localization of rab-7 and TRP-1 in the melanosomes with early stages of maturation (I-HI). Furthermore, this process will also be regulated by phosphatidylinositol 3-kinase (PI-3 kinase).

Aberrant retention of tyrosinase in the endoplasmic reticulum mediates accelerated degradation of the enzyme and contributes to the dedifferentiated phenotype of amelanotic melanoma cells

The loss of tyrosinase, the key enzyme in melanin synthesis, has been implicated in the dedifferentiation of malignant melanocytes. The presence of tyrosinase transcripts and antigenic peptides in melanoma tumors prompted us to investigate whether the basis for the loss of the enzyme was proteolytic degradation. Toward this aim, we followed the kinetics of synthesis, degradation, processing, chaperone binding, inhibitor sensitivity, and subcellular localization of tyrosinase in normal and malignant melanocytes. We found that, in amelanotic melanoma cell lines, tyrosinase failed to reach the melanosome, the organelle for melanin synthesis, because it was retained in the endoplasmic reticulum (ER) and then degraded. Tyrosinase appeared mostly as a 70-kDa core-glycosylated, endoglycosidase H-sensitive, immature form bound to the ER chaperone calnexin and had a life-span of only 25% of normal. Maturation and transit from the ER to the Golgi compartment was facilitated by lowering the temperature of incubation to 31 degrees C. Several proteasome inhibitors caused the accumulation of an approximately 60-kDa tyrosinase doublet that was more prominent in malignant than in normal melanocytes and promoted, to various degrees, the maturation of tyrosinase in melanoma cells and the translocation of the enzyme to melanosomes. The appearance of ubiquitinated tyrosinase after treatment of normal melanocytes with N-acetyl-L-leucinyl-L-leucinal-L-norleucinal reinforced our notion that some tyrosinase is normally degraded by proteasomes. Proteolysis of tyrosinase by proteasomes is consistent with the production of antigenic tyrosinase peptides that are presented to the immune system by major histocompatibility complex class I molecules.

Myosin V associates with melanosomes in mouse melanocytes: evidence that myosin V is an organelle motor

Mice with mutations at the dilute locus exhibit a ‘washed out’ or ‘diluted’ coat color. The pigments that are responsible for the coloration of mammalian hair are produced by melanocytes within a specialized organelle, the melanosome. Each melanocyte is responsible for delivering melanosomes via its extensive dendritic arbor to numerous keratinocytes, which go on to form the pigmented hair shaft. In this study, we show by light immunofluorescence microscopy and immunoelectron microscopy that the myosin V isoform encoded by the dilute locus associates with melanosomes. This association, which was seen in all mouse melanocyte cell lines examined and with two independent myosin V antibodies, was evident not only within completely melanized cells, but also within cells undergoing the process of melanosome biogenesis, where coordinate changes in the distributions of a melanosome marker and myosin V were seen. To determine where myosin V, a known actin-based motor, might play a role in melanosome transport, we also examined the cellular distribution of F-actin. The only region where myosin V and F-actin were both concentrated was in dendrites and dendritic tips, which represent the sole destination for melanosomes and where they accumulate in cultured melanocytes. These results support the idea that myosin V serves as the motor for the outward movement of melanosomes within dendritic extensions, and, together with the available information regarding the phenotype of mutant melanocytes in vitro, argue that coat color dilution is caused by the absense of this myosin V-dependent melanosome transport system.

An alpha-helical signal in the cytosolic domain of the interleukin 2 receptor beta chain mediates sorting towards degradation after endocytosis

High-affinity IL2 receptors consist of three components, the alpha, beta, and gamma chains that are associated in a noncovalent manner. Both the beta and gamma chains belong to the cytokine receptor superfamily. Interleukin 2 (IL2) binds to high-affinity receptors on the cell surface and IL2-receptor complexes are internalized. After endocytosis, the components of this multimolecular receptor have different intracellular fates: one of the chains, alpha, recycles to the plasma membrane, while the others, beta and gamma, are routed towards late endocytic compartments and are degraded. We show here that the cytosolic domain of the beta chain contains a 10-amino acid sequence which codes for a sorting signal. When transferred to a normally recycling receptor, this sequence diverts it from recycling. The structure of a 17-amino acid segment of the beta chain including this sequence has been studied by nuclear magnetic resonance and circular dichroism spectroscopy, which revealed that the 10 amino acids corresponding to the sorting signal form an amphipathic alpha helix. This work thus describes a novel, highly structured signal, which is sufficient for sorting towards degradation compartments after endocytosis.

Immune response to a differentiation antigen induced by altered antigen: a study of tumor rejection and autoimmunity

Recognition of self is emerging as a theme for the immune recognition of human cancer. One question is whether the immune system can actively respond to normal tissue autoantigens expressed by cancer cells. A second but related question is whether immune recognition of tissue autoantigens can actually induce tumor rejection. To address these issues, a mouse model was developed to investigate immune responses to a melanocyte differentiation antigen, tyrosinase-related protein 1 (or gp75), which is the product of the brown locus. In mice, immunization with purified syngeneic gp75 or syngeneic cells expressing gp75 failed to elicit antibody or cytotoxic T-cell responses to gp75, even when different immune adjuvants and cytokines were included. However, immunization with altered sources of gp75 antigen, in the form of either syngeneic gp75 expressed in insect cells or human gp75, elicited autoantibodies to gp75. Immunized mice rejected metastatic melanomas and developed patchy depigmentation in their coats. These studies support a model of tolerance maintained to a melanocyte differentiation antigen where tolerance can be broken by presenting sources of altered antigen (e.g., homologous xenogeneic protein or protein expressed in insect cells). Immune responses induced with these sources of altered antigen reacted with various processed forms of native, syngeneic protein and could induce both tumor rejection and autoimmunity.

The ocular albinism type 1 gene product is a membrane glycoprotein localized to melanosomes

Ocular albinism type 1 (OA1) is an inherited disorder characterized by severe reduction of visual acuity, photophobia, and retinal hypopigmentation. Ultrastructural examination of skin melanocytes and of the retinal pigment epithelium reveals the presence of macromelanosomes, suggesting a defect in melanosome biogenesis. The gene responsible for OA1 is exclusively expressed in pigment cells and encodes a predicted protein of 404 aa displaying several putative transmembrane domains and sharing no similarities with previously identified molecules. Using polyclonal antibodies we have identified the endogenous OA1 protein in retinal pigment epithelial cells, in normal human melanocytes and in various melanoma cell lines. Two forms of the OA1 protein were identified by Western analysis, a 60-kDa glycoprotein and a doublet of 48 and 45 kDa probably corresponding to unglycosylated precursor polypeptides. Upon subcellular fractionation and phase separation with the nonionic detergent Triton X-114, the OA1 protein segregated into the melanosome-rich fraction and behaved as an authentic integral membrane protein. Immunofluorescence and immunogold analyses on normal human melanocytes confirmed the melanosomal membrane localization of the endogenous OA1 protein, consistent with its possible involvement in melanosome biogenesis. The identification of a novel melanosomal membrane protein involved in a human disease will provide insights into the mechanisms that control the cell-specific pathways of subcellular morphogenesis.

Melanoma-specific CD4+ T cells recognize nonmutated HLA-DR-restricted tyrosinase epitopes

Tyrosinase was the first melanoma-associated antigen shown to be recognized by CD4+ T cells. In this study, we have identified two HLA-DRB1*0401-restricted peptides recognized by these T cells: Ty 56-70 and Ty 448-462. As with many of the MHC class I-restricted melanoma epitopes, both are nonmutated self peptides that have intermediate and weak MHC binding affinities, respectively. Mutated and truncated versions of these peptides were used to define their MHC binding anchor residues. Anchor residues were then modified to derive peptides with increased MHC binding affinities and T cell stimulatory properties. Ty 56-70 and Ty 448-462 enhance the list of immunogenic HLA-A2-, A24-, and B44-restricted tyrosinase peptides already described. Thus, tyrosinase provides a model for anti-melanoma vaccines in which a single molecule can generate multivalent immunization incorporating both CD4+ and CD8+ T cell responses.

Characterization and subcellular localization of human Pmel 17/silver, a 110-kDa (pre)melanosomal membrane protein associated with 5,6,-dihydroxyindole-2-carboxylic acid (DHICA) converting activity

Pmel 17 is preferentially expressed in pigment cells in a manner suggestive of involvement in melanin biosynthesis. The gene is identical to the silver (si) pigmentation locus in mice. We now produced a recombinant glutathione-S-transferase-human Pmel 17 infusion protein and raised polyclonal antibodies against it to confirm the ultrastructural location and presumed site of action predicted by the deduced primary structure of Pmel 17/silver, and to authenticate the specificity of the DHICA converting function as inherent to the silver-locus protein. Full-length Pmel 17 cDNA also produced in insect cells in a baculovirus expression vector to ensure that activity did not originate from a co-precipitated protein. Natural hPmel 17 from human melanoma cells has an approximate molecular size of 100 kDa. By immunoperoxidase electron microscopic cytochemistry, the antigen was localized to the limiting membranes of premelanosomes and presumed premelanogenic cytosolic vesicles and, to a minor extent, in the premelanosomal matrix. In an in vitro assay, both the natural and the recombinant Pmel 17 accelerated the conversion of DHICA to melanin. This activity was inhibited by the anti-Pmel 17 polyclonal antibodies, indicating that the acceleration of DHICA conversion by natural protein is genuine and cannot be due to contaminating complexed proteins. We suggest that in situ Pmel 17/silver is a component of a postulated premelanosomal/melanosomal complex of membrane-bound melanogenic oxidoreductive enzymes and cofactors, in analogy to the electron transfer chain in mitochondria.