Sialylated core 1 O-glycans influence the sorting of Pmel17/gp100 and determine its capacity to form fibrils

Factors influencing amyloid fibril formation

Protein aggregation is a complex process with several stages that lead to the formation of complex structures and shapes with a broad variability in stability and toxicity. The aggregation process is affected by various factors and environmental conditions that disrupt the protein's original state, including internal factors like mutations, expression levels, and polypeptide chain truncation, as well as external factors, such as dense molecular surroundings, post-translation modifications, and interactions with other proteins, nucleic acids, small molecules, metal ions, chaperones, and lipid membranes. During the aggregation process, the biological activity of an aggregating protein may be reduced or eliminated, whereas the resulting aggregates may have the potential to be immunogenic, or they may have other undesirable properties. Finding the cause(s) of protein aggregation and controlling it to an acceptable level is among the most crucial topics of research in academia and biopharmaceutical companies. This chapter aims to review intrinsic pathways of protein aggregation and potential extrinsic variables that influence this process.

Sudocetaxel Zendusortide (TH1902) triggers the cGAS/STING pathway and potentiates anti-PD-L1 immune-mediated tumor cell killing

The anticancer efficacy of Sudocetaxel Zendusortide (TH1902), a peptide-drug conjugate internalized through a sortilin-mediated process, was assessed in a triple-negative breast cancer-derived MDA-MB-231 immunocompromised xenograft tumor model where complete tumor regression was observed for more than 40 days after the last treatment. Surprisingly, immunohistochemistry analysis revealed high staining of STING, a master regulator in the cancer-immunity cycle. A weekly administration of TH1902 as a single agent in a murine B16-F10 melanoma syngeneic tumor model demonstrated superior tumor growth inhibition than did docetaxel. A net increase in CD45 leukocyte infiltration within TH1902-treated tumors, especially for tumor-infiltrating lymphocytes and tumor-associated macrophages was observed. Increased staining of perforin, granzyme B, and caspase-3 was suggestive of elevated cytotoxic T and natural killer cell activities. Combined TH1902/anti-PD-L1 treatment led to increases in tumor growth inhibition and median animal survival. TH1902 inhibited cell proliferation and triggered apoptosis and senescence in B16-F10 cells in vitro, while inducing several downstream effectors of the cGAS/STING pathway and the expression of MHC-I and PD-L1. This is the first evidence that TH1902 exerts its antitumor activity, in part, through modulation of the immune tumor microenvironment and that the combination of TH1902 with checkpoint inhibitors (anti-PD-L1) could lead to improved clinical outcomes.

Disrupting the Repeat Domain of Premelanosome Protein (PMEL) Produces Dysamyloidosis and Dystrophic Ocular Pigment Reflective of Pigmentary Glaucoma

Pigmentary glaucoma has recently been associated with missense mutations in PMEL that are dominantly inherited and enriched in the protein's fascinating repeat domain. PMEL pathobiology is intriguing because PMEL forms functional amyloid in healthy eyes, and this PMEL amyloid acts to scaffold melanin deposition. This is an informative contradistinction to prominent neurodegenerative diseases where amyloid formation is neurotoxic and mutations cause a toxic gain of function called "amyloidosis". Preclinical animal models have failed to model this PMEL "dysamyloidosis" pathomechanism and instead cause recessively inherited ocular pigment defects via PMEL loss of function; they have not addressed the consequences of disrupting PMEL's repetitive region. Here, we use CRISPR to engineer a small in-frame mutation in the zebrafish homolog of PMEL that is predicted to subtly disrupt the protein's repetitive region. Homozygous mutant larvae displayed pigmentation phenotypes and altered eye morphogenesis similar to presumptive null larvae. Heterozygous mutants had disrupted eye morphogenesis and disrupted pigment deposition in their retinal melanosomes. The deficits in the pigment deposition of these young adult fish were not accompanied by any detectable glaucomatous changes in intraocular pressure or retinal morphology. Overall, the data provide important in vivo validation that subtle PMEL mutations can cause a dominantly inherited pigment pathology that aligns with the inheritance of pigmentary glaucoma patient pedigrees. These in vivo observations help to resolve controversy regarding the necessity of PMEL's repeat domain in pigmentation. The data foster an ongoing interest in an antithetical dysamyloidosis mechanism that, akin to the amyloidosis of devastating dementias, manifests as a slow progressive neurodegenerative disease.

PMEL is mutated in oculocutaneous albinism

Oculocutaneous albinism (OCA) is a group of Mendelian disorders characterized by hypopigmentation of skin, hair and pigmented ocular structures. While much of the genetic heterogeneity of OCA has been resolved, many patients still lack a molecular diagnosis following exome sequencing. Here, we report a consanguineous family in which the index patient presented with OCA and Hirschsprung disease but tested negative for known genetic causes of OCA. Instead, he was found to have a homozygous presumptive loss of function variant in PMEL. PMEL encodes a scaffolding protein that is essential for the normal maturation of melanosomes and normal deposition of the melanin pigment therein. Numerous PMEL vertebrate ortholog mutants have been reported and all were characterized by conspicuous pigmentary abnormalities. We suggest that the patient we report is the first human equivalent of PMEL loss of function.

Protein Aggregation and Disaggregation in Cells and Development

Protein aggregation is a feature of numerous neurodegenerative diseases. However, regulated, often reversible, formation of protein aggregates, also known as condensates, helps control a wide range of cellular activities including stress response, gene expression, memory, cell development and differentiation. This review presents examples of aggregates found in biological systems, how they are used, and cellular strategies that control aggregation and disaggregation. We include features of the aggregating proteins themselves, environmental factors, co-aggregates, post-translational modifications and well-known aggregation-directed activities that influence their formation, material state, stability and dissolution. We highlight the emerging roles of biomolecular condensates in early animal development, and disaggregation processing proteins that have recently been shown to play key roles in gametogenesis and embryogenesis.

Functional Domains and Evolutionary History of the PMEL and GPNMB Family Proteins

The ancient paralogs premelanosome protein (PMEL) and glycoprotein nonmetastatic melanoma protein B (GPNMB) have independently emerged as intriguing disease loci in recent years. Both proteins possess common functional domains and variants that cause a shared spectrum of overlapping phenotypes and disease associations: melanin-based pigmentation, cancer, neurodegenerative disease and glaucoma. Surprisingly, these proteins have yet to be shown to physically or genetically interact within the same cellular pathway. This juxtaposition inspired us to compare and contrast this family across a breadth of species to better understand the divergent evolutionary trajectories of two related, but distinct, genes. In this study, we investigated the evolutionary history of PMEL and GPNMB in clade-representative species and identified TMEM130 as the most ancient paralog of the family. By curating the functional domains in each paralog, we identified many commonalities dating back to the emergence of the gene family in basal metazoans. PMEL and GPNMB have gained functional domains since their divergence from TMEM130, including the core amyloid fragment (CAF) that is critical for the amyloid potential of PMEL. Additionally, the PMEL gene has acquired the enigmatic repeat domain (RPT), composed of a variable number of imperfect tandem repeats; this domain acts in an accessory role to control amyloid formation. Our analyses revealed the vast variability in sequence, length and repeat number in homologous RPT domains between craniates, even within the same taxonomic class. We hope that these analyses inspire further investigation into a gene family that is remarkable from the evolutionary, pathological and cell biology perspectives.

Identification of critical amino acid residues in the regulatory N-terminal domain of PMEL

The pigment cell-specific protein PMEL forms a functional amyloid matrix in melanosomes onto which the pigment melanin is deposited. The amyloid core consists of a short proteolytic fragment, which we have termed the core-amyloid fragment (CAF) and perhaps additional parts of the protein, such as the PKD domain. A highly O-glycosylated repeat (RPT) domain also derived from PMEL proteolysis associates with the amyloid and is necessary to establish the sheet-like morphology of the assemblies. Excluded from the aggregate is the regulatory N-terminus, which nevertheless must be linked in cis to the CAF in order to drive amyloid formation. The domain is then likely cleaved away immediately before, during, or immediately after the incorporation of a new CAF subunit into the nascent amyloid. We had previously identified a 21 amino acid long region, which mediates the regulatory activity of the N-terminus towards the CAF. However, many mutations in the respective segment caused misfolding and/or blocked PMEL export from the endoplasmic reticulum, leaving their phenotype hard to interpret. Here, we employ a saturating mutagenesis approach targeting the motif at single amino acid resolution. Our results confirm the critical nature of the PMEL N-terminal region and identify several residues essential for PMEL amyloidogenesis.

Functional Mammalian Amyloids and Amyloid-Like Proteins

Amyloids are highly ordered fibrous cross-β protein aggregates that are notorious primarily because of association with a variety of incurable human and animal diseases (termed amyloidoses), including Alzheimer’s disease (AD), Parkinson’s disease (PD), type 2 diabetes (T2D), and prion diseases. Some amyloid-associated diseases, in particular T2D and AD, are widespread and affect hundreds of millions of people all over the world. However, recently it has become evident that many amyloids, termed “functional amyloids,” are involved in various activities that are beneficial to organisms. Functional amyloids were discovered in diverse taxa, ranging from bacteria to mammals. These amyloids are involved in vital biological functions such as long-term memory, storage of peptide hormones and scaffolding melanin polymerization in animals, substrate attachment, and biofilm formation in bacteria and fungi, etc. Thus, amyloids undoubtedly are playing important roles in biological and pathological processes. This review is focused on functional amyloids in mammals and summarizes approaches used for identifying new potentially amyloidogenic proteins and domains.

Melatonin exerts oncostatic capacity and decreases melanogenesis in human MNT-1 melanoma cells

Melanogenesis is a key parameter of differentiation in melanocytes and melanoma cells; therefore, search for factors regulating this pathway are strongly desired. Herein, we investigated the effects of melatonin, a ubiquitous physiological mediator that is found throughout animals and plants. In mammals, the pineal gland secretes this indoleamine into the blood circulation to exert an extensive repertoire of biological activities. Our in vitro assessment indicates an oncostatic capacity of melatonin in time-dependent manner (24, 48, 72 hours) in highly pigmented MNT-1 melanoma cells. The similar pattern of regulation regarding cell viability was observed in amelanotic Sk-Mel-28 cells. Subsequently, MNT-1 cells were tested for the first time for evaluation of melanin/melatonin interaction. Thus primary, electron paramagnetic resonance (EPR) spectroscopy demonstrated that melatonin reduced melanin content. Artificially induced disturbances of melanogenesis by selected inhibitors (N-phenylthiourea or kojic acid) were slightly antagonized by melatonin. Additionally, analysis using transmission electron microscopy has shown that melatonin, particularly at higher dose of 10-3  mol/L, triggered the appearance of premelanosomes (stage I-II of melanosome) and MNT-1 cells synthesize de novo endogenous melatonin shown by LC-MS. In conclusion, these studies show a melanogenic-like function of melatonin suggesting it as an advantageous agent for treatment of pigmentary disorders.

Non-Synonymous variants in premelanosome protein (PMEL) cause ocular pigment dispersion and pigmentary glaucoma

Pigmentary glaucoma (PG) is a common glaucoma subtype that results from release of pigment from the iris, called pigment dispersion syndrome (PDS), and its deposition throughout the anterior chamber of the eye. Although PG has a substantial heritable component, no causative genes have yet been identified. We used whole exome sequencing of two independent pedigrees to identify two premelanosome protein (PMEL) variants associated with heritable PDS/PG. PMEL encodes a key component of the melanosome, the organelle essential for melanin synthesis, storage and transport. Targeted screening of PMEL in three independent cohorts (n = 394) identified seven additional PDS/PG-associated non-synonymous variants. Five of the nine variants exhibited defective processing of the PMEL protein. In addition, analysis of PDS/PG-associated PMEL variants expressed in HeLa cells revealed structural changes to pseudomelanosomes indicating altered amyloid fibril formation in five of the nine variants. Introduction of 11-base pair deletions to the homologous pmela in zebrafish by the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 method caused profound pigmentation defects and enlarged anterior segments in the eye, further supporting PMEL's role in ocular pigmentation and function. Taken together, these data support a model in which missense PMEL variants represent dominant negative mutations that impair the ability of PMEL to form functional amyloid fibrils. While PMEL mutations have previously been shown to cause pigmentation and ocular defects in animals, this research is the first report of mutations in PMEL causing human disease.

Repeat domain-associated O-glycans govern PMEL fibrillar sheet architecture

PMEL is a pigment cell-specific protein that forms a functional amyloid matrix in melanosomes. The matrix consists of well-separated fibrillar sheets on which the pigment melanin is deposited. Using electron tomography, we demonstrate that this sheet architecture is governed by the PMEL repeat (RPT) domain, which associates with the amyloid as an accessory proteolytic fragment. Thus, the RPT domain is dispensable for amyloid formation as such but shapes the morphology of the matrix, probably in order to maximize the surface area available for pigment adsorption. Although the primary amino acid sequence of the RPT domain differs vastly among various vertebrates, we show that it is a functionally conserved, interchangeable module. RPT domains of all species are predicted to be very highly O-glycosylated, which is likely the common defining feature of this domain. O-glycosylation is indeed essential for RPT domain function and the establishment of the PMEL sheet architecture. Thus, O-glycosylation, not amino acid sequence, appears to be the major factor governing the characteristic PMEL amyloid morphology.

pH-Dependent fibril maturation of a Pmel17 repeat domain isoform revealed by tryptophan fluorescence

The pre-melanosomal protein (Pmel17) aggregates within melanosomes to form functional amyloid fibrils that facilitate melanin polymerization. The repeat domain (RPT) of Pmel17 fibrillates under strict acidic melanosomal pH. Alternative splicing results in a shortened repeat domain (sRPT), which also forms amyloid fibrils. Here, we explored the effects of pH and protein concentration on sRPT aggregation by monitoring the intrinsic fluorescence of the sole tryptophan at position 381 (³⁸¹W). ³⁸¹W emission properties revealed changes of local environment polarity for sRPT fibrils formed at different pH. At pH 4, fibrils formed rapidly with no lag phase. A high ³⁸¹W intensity was observed with a slight blue shift (10 nm). These fibrils underwent further structural rearrangements at intermediate pH (5-6), mirroring that of melanosome maturation, which initiates at pH 4 and increases to near neutral pH. In contrast, typical sigmoidal kinetics were observed at pH 6 with slower rates and ³⁸¹W exhibited quenched emission. Interestingly, biphasic kinetics were observed at pH 5 in a protein concentration-dependent manner. A large ³⁸¹W blue shift (23 nm) was measured, indicating a more hydrophobic environment for fibrils made at pH 5. Consistent with ³⁸¹W fluorescence, Raman spectroscopy revealed molecular level perturbations in sRPT fibrils that were not evident from circular dichroism, transmission electron microscopy, or limited proteolysis analysis. Finally, sRPT fibrils did not form at pH ≥7 and preformed fibrils rapidly disaggregated under these solution conditions. Collectively, this work yields mechanistic insights into pH-dependent sRPT aggregation in the context of melanosome maturation.

Nonsense mutation in PMEL is associated with yellowish plumage colour phenotype in Japanese quail

The L strain of Japanese quail exhibits a plumage phenotype that is light yellowish in colour. In this study, we identified a nonsense mutation in the premelanosome protein (PMEL) gene showing complete concordance with the yellowish plumage within a pedigree as well as across strains by genetic linkage analysis of an F₂ intercross population using approximately 2,000 single nucleotide polymorphisms (SNPs) that were detected by double digest restriction site-associated DNA sequencing (ddRAD-seq). The yellowish plumage was inherited in an autosomal recessive manner, and the causative mutation was located within an 810-kb genomic region of the LGE22C19W28_E50C23 linkage group (LGE22). This region contained the PMEL gene that is required for the normal melanosome morphogenesis and eumelanin deposition. A nonsense mutation that leads to a marked truncation of the deduced protein was found in PMEL of the mutant. The gene expression level of PMEL decreased substantially in the mutant. Genotypes at the site of the nonsense mutation were fully concordant with plumage colour phenotypes in 196 F₂ offspring. The nonsense mutation was not found in several quail strains with non-yellowish plumage. Thus, the yellowish plumage may be caused by the reduced eumelanin content in feathers because of the loss of PMEL function.

Why Study Functional Amyloids? Lessons from the Repeat Domain of Pmel17

One of the current challenges facing biomedical researchers is the need to develop new approaches in preventing amyloid formation that is associated with disease. While amyloid is generally considered detrimental to the cell, examples of amyloids that maintain a benign nature and serve a specific function exist. Here, we review our work on the repeat domain (RPT) of the functional amyloid Pmel17. Specifically, the RPT domain contributes in generating amyloid fibrils in melanosomes upon which melanin biosynthesis occurs. Amyloid formation of RPT was shown to be pH sensitive, aggregating only under acidic conditions associated with melanosomal pH. Furthermore, preformed fibrils rapidly dissolved at neutral pH to generate benign monomeric species. From a biological perspective, this unique reversible aggregation/disaggregation is a safeguard against an event of releasing RPT fibrils in the cytosol, resulting in rapid fibril unfolding and circumventing cytotoxicity. Understanding how melanosomes preserve a safe environment will address vital questions that remain unanswered with pathological amyloids.

Reversing the amyloid trend: Mechanism of fibril assembly and dissolution of the repeat domain from a human functional amyloid

Amyloids are traditionally observed in the context of disease. However, there is growing momentum that these structures can serve a beneficial role where the amyloid carries out a specific function. These so called 'functional amyloids' have all the structural hallmarks of disease-associated amyloids, raising the question as to what differentiates a well-behaved benign amyloid from a lethally destructive one. Here, we review our work on the repeat domain (RPT) from Pmel17, an important functional amyloid involved in melanin biosynthesis. Particularly, we focused our attention on the unique reversible aggregation-disaggregation process of RPT that is controlled strictly by solution pH. This pH dependence of RPT amyloid formation functions as a switch to control fibril assembly and maintains the benign nature that is associated with functional amyloids.

PMEL Amyloid Fibril Formation: The Bright Steps of Pigmentation

In pigment cells, melanin synthesis takes place in specialized organelles, called melanosomes. The biogenesis and maturation of melanosomes is initiated by an unpigmented step that takes place prior to the initiation of melanin synthesis and leads to the formation of luminal fibrils deriving from the pigment cell-specific pre-melanosomal protein (PMEL). In the lumen of melanosomes, PMEL fibrils optimize sequestration and condensation of the pigment melanin. Interestingly, PMEL fibrils have been described to adopt a typical amyloid-like structure. In contrast to pathological amyloids often associated with neurodegenerative diseases, PMEL fibrils represent an emergent category of physiological amyloids due to their beneficial cellular functions. The formation of PMEL fibrils within melanosomes is tightly regulated by diverse mechanisms, such as PMEL traffic, cleavage and sorting. These mechanisms revealed increasing analogies between the formation of physiological PMEL fibrils and pathological amyloid fibrils. In this review we summarize the known mechanisms of PMEL fibrillation and discuss how the recent understanding of physiological PMEL amyloid formation may help to shed light on processes involved in pathological amyloid formation.

A β-solenoid model of the Pmel17 repeat domain: insights to the formation of functional amyloid fibrils

Pmel17 is a multidomain protein involved in biosynthesis of melanin. This process is facilitated by the formation of Pmel17 amyloid fibrils that serve as a scaffold, important for pigment deposition in melanosomes. A specific luminal domain of human Pmel17, containing 10 tandem imperfect repeats, designated as repeat domain (RPT), forms amyloid fibrils in a pH-controlled mechanism in vitro and has been proposed to be essential for the formation of the fibrillar matrix. Currently, no three-dimensional structure has been resolved for the RPT domain of Pmel17. Here, we examine the structure of the RPT domain by performing sequence threading. The resulting model was subjected to energy minimization and validated through extensive molecular dynamics simulations. Structural analysis indicated that the RPT model exhibits several distinct properties of β-solenoid structures, which have been proposed to be polymerizing components of amyloid fibrils. The derived model is stabilized by an extensive network of hydrogen bonds generated by stacking of highly conserved polar residues of the RPT domain. Furthermore, the key role of invariant glutamate residues is proposed, supporting a pH-dependent mechanism for RPT domain assembly. Conclusively, our work attempts to provide structural insights into the RPT domain structure and to elucidate its contribution to Pmel17 amyloid fibril formation.

The Kringle-like Domain Facilitates Post-endoplasmic Reticulum Changes to Premelanosome Protein (PMEL) Oligomerization and Disulfide Bond Configuration and Promotes Amyloid Formation

The formation of functional amyloid must be carefully regulated to prevent the accumulation of potentially toxic products. Premelanosome protein (PMEL) forms non-toxic functional amyloid fibrils that assemble into sheets upon which melanins ultimately are deposited within the melanosomes of pigment cells. PMEL is synthesized in the endoplasmic reticulum but forms amyloid only within post-Golgi melanosome precursors; thus, PMEL must traverse the secretory pathway in a non-amyloid form. Here, we identified two pre-amyloid PMEL intermediates that likely regulate the timing of fibril formation. Analyses by non-reducing SDS-PAGE, size exclusion chromatography, and sedimentation velocity revealed two native high Mr disulfide-bonded species that contain Golgi-modified forms of PMEL. These species correspond to disulfide bond-containing dimeric and monomeric PMEL isoforms that contain no other proteins as judged by two-dimensional PAGE of metabolically labeled/immunoprecipitated PMEL and by mass spectrometry of affinity-purified complexes. Metabolic pulse-chase analyses, small molecule inhibitor treatments, and evaluation of site-directed mutants suggest that the PMEL dimer forms around the time of endoplasmic reticulum exit and is resolved by disulfide bond rearrangement into a monomeric form within the late Golgi or a post-Golgi compartment. Mutagenesis of individual cysteine residues within the non-amyloid cysteine-rich Kringle-like domain stabilizes the disulfide-bonded dimer and impairs fibril formation as determined by electron microscopy. Our data show that the Kringle-like domain facilitates the resolution of disulfide-bonded PMEL dimers and promotes PMEL functional amyloid formation, thereby suggesting that PMEL dimers must be resolved to monomers to generate functional amyloid fibrils.

Podoplanin requires sialylated O-glycans for stable expression on lymphatic endothelial cells and for interaction with platelets

O-glycosylation of podoplanin (PDPN) on lymphatic endothelial cells is critical for the separation of blood and lymphatic systems by interacting with platelet C-type lectin-like receptor 2 during development. However, how O-glycosylation controls endothelial PDPN function and expression remains unclear. In this study, we report that core 1 O-glycan-deficient or desialylated PDPN was highly susceptible to proteolytic degradation by various proteases, including metalloproteinases (MMP)-2/9. We found that the lymph contained activated MMP-2/9 and incubation of the lymph reduced surface levels of PDPN on core 1 O-glycan-deficient endothelial cells, but not on wild-type ECs. The lymph from mice with sepsis induced by cecal ligation and puncture, which contained bacteria-derived sialidase, reduced PDPN levels on wild-type ECs. The MMP inhibitor, GM6001, rescued these reductions. Additionally, GM6001 treatment rescued the reduction of PDPN level on lymphatic endothelial cells in mice lacking endothelial core 1 O-glycan or cecal ligation and puncture-treated mice. Furthermore, core 1 O-glycan-deficient or desialylated PDPN impaired platelet interaction under physiological flow. These data indicate that sialylated O-glycans of PDPN are essential for platelet adhesion and prevent PDPN from proteolytic degradation primarily mediated by MMPs in the lymph.

The PKD domain distinguishes the trafficking and amyloidogenic properties of the pigment cell protein PMEL and its homologue GPNMB

Proteolytic fragments of the pigment cell-specific glycoprotein, PMEL, form the amyloid fibrillar matrix underlying melanins in melanosomes. The fibrils form within multivesicular endosomes to which PMEL is selectively sorted and that serve as melanosome precursors. GPNMB is a tissue-restricted glycoprotein with substantial sequence homology to PMEL, but no known function, and was proposed to localize to non-fibrillar domains of distinct melanosome subcompartments in melanocytes. Here we confirm that GPNMB localizes to compartments distinct from the PMEL-containing multivesicular premelanosomes or late endosomes in melanocytes and HeLa cells, respectively, and is largely absent from fibrils. Using domain swapping, the unique PMEL localization is ascribed to its polycystic kidney disease (PKD) domain, whereas the homologous PKD domain of GPNMB lacks apparent sorting function. The difference likely reflects extensive modification of the GPNMB PKD domain by N-glycosylation, nullifying its sorting function. These results reveal the molecular basis for the distinct trafficking and morphogenetic properties of PMEL and GPNMB and support a deterministic function of the PMEL PKD domain in both protein sorting and amyloidogenesis.

PMEL: a pigment cell-specific model for functional amyloid formation

PMEL is a pigment cell-specific protein responsible for the formation of fibrillar sheets within the pigment organelle, the melanosome. The fibrillar sheets serve as a template upon which melanins polymerize as they are synthesized. The PMEL fibrils are required for optimal pigment cell function, as animals that either lack PMEL expression or express mutant PMEL variants show varying degrees of hypopigmentation and pigment cell inviability. The PMEL fibrils have biophysical properties of amyloid, a protein fold that is frequently associated with neurodegenerative and other diseases. However, PMEL is one of a growing number of non-pathogenic amyloid proteins that contribute to the function of the cell and/or organism that produces them. Understanding how PMEL generates amyloid in a non-pathogenic manner might provide insights into how to avoid toxicity due to pathological amyloid formation. In this review, we summarize and reconcile data concerning the fate of PMEL from its site of synthesis in the endoplasmic reticulum to newly formed melanosomes and the role of distinct PMEL subdomains in trafficking and amyloid fibril formation. We then discuss how its progression through the secretory pathway into the endosomal system might allow for the regulated and non-toxic conversion of PMEL into an ordered amyloid polymer.

Mechanisms of protein delivery to melanosomes in pigment cells

Vertebrate pigment cells in the eye and skin are useful models for cell types that use specialized endosomal trafficking pathways to partition cargo proteins to unique lysosome-related organelles such as melanosomes. This review describes current models of protein trafficking required for melanosome biogenesis in mammalian melanocytes.

The yin and yang of amyloid: insights from α-synuclein and repeat domain of Pmel17

Amyloid has been traditionally viewed in the context of disease. However, the emerging concept of 'functional amyloid' has taken a new direction into how we view amyloid. Recent studies have identified amyloid fibrils ranging from bacteria to humans that have a beneficial role, instead of being associated with a misfolded state that has been implicated in diseases such as Alzheimer's, Parkinson's and prion diseases. Here, we review our work on two human amyloidogenic polypeptides, one associated with Parkinson's disease, α-synuclein (α-syn), and the other important for melanin synthesis, the repeat domain (RPT) from Pmel17. Particularly, we focused our attention on spectroscopic studies of protein conformation and dynamics and their impact on α-syn amyloid formation and for RPT, we discussed the strict pH dependence of amyloid formation and its role in melanin biosynthesis.

Repeat domains of melanosome matrix protein Pmel17 orthologs form amyloid fibrils at the acidic melanosomal pH

Most amyloids are pathological, but fragments of Pmel17 form a functional amyloid in vertebrate melanosomes essential for melanin synthesis and deposition. We previously reported that only at the mildly acidic pH (4-5.5) typical of melanosomes, the repeat domain (RPT) of human Pmel17 can form amyloid in vitro. Combined with the known presence of RPT in the melanosome filaments and the requirement of this domain for filament formation, we proposed that RPT may be the core of the amyloid formed in vivo. Although most of Pmel17 is highly conserved across a broad range of vertebrates, the RPT domains vary dramatically, with no apparent homology in some cases. Here, we report that the RPT domains of mouse and zebrafish, as well as a small splice variant of human Pmel17, all form amyloid specifically at mildly acid pH (pH ∼5.0). Protease digestion, mass per unit length measurements, and solid-state NMR experiments suggest that amyloid of the mouse RPT has an in-register parallel β-sheet architecture with two RPT molecules per layer, similar to amyloid of the Aβ peptide. Although there is no sequence conservation between human and zebrafish RPT, amyloid formation at acid pH is conserved.

Proprotein convertases process Pmel17 during secretion

Pmel17 is a melanocyte/melanoma-specific protein that traffics to melanosomes where it forms a fibrillar matrix on which melanin gets deposited. Before being cleaved into smaller fibrillogenic fragments the protein undergoes processing by proprotein convertases, a class of serine proteases that typically recognize the canonical motif RX(R/K)R↓. The current model of Pmel17 maturation states that this processing step occurs in melanosomes, but in light of recent reports this issue has become controversial. We therefore addressed this question by thoroughly assessing the processing kinetics of either wild-type Pmel17 or a secreted soluble Pmel17 derivative. Our results demonstrate clearly that processing of Pmel17 occurs during secretion and that it does not require entry of the protein into the endocytic system. Strikingly, processing proceeds even in the presence of the secretion inhibitor monensin, suggesting that Pmel17 is an exceptionally good substrate. In line with this, we find that newly synthesized surface Pmel17 is already quantitatively cleaved. Moreover, we demonstrate that Pmel17 function is independent of the sequence identity of its unconventional proprotein convertase-cleavage motif that lacks arginine in P4 position. The data alter the current view of Pmel17 maturation and suggest that the multistep processing of Pmel17 begins with an early cleavage during secretion that primes the protein for later functional processing.

Effects of pH on aggregation kinetics of the repeat domain of a functional amyloid, Pmel17

Pmel17 is a functional amyloidogenic protein whose fibrils act as scaffolds for pigment deposition in human skin and eyes. We have used the repeat domain (RPT, residues 315-444), an essential luminal polypeptide region of Pmel17, as a model system to study conformational changes from soluble unstructured monomers to β-sheet-containing fibrils. Specifically, we report on the effects of solution pH (4 → 7) mimicking pH conditions of melanosomes, acidic organelles where Pmel17 fibrils are formed. Local, secondary, and fibril structure were monitored via intrinsic Trp fluorescence, circular dichroism spectroscopy, and transmission electron microscopy, respectively. We find that W423 is a highly sensitive probe of amyloid assembly with spectral features reflecting local conformational and fibril morphological changes. A critical pH regime (5 ± 0.5) was identified for fibril formation suggesting the involvement of at least three carboxylic acids in the structural rearrangement necessary for aggregation. Moreover, we demonstrate that RPT fibril morphology can be transformed directly by changing solution pH. Based on these results, we propose that intramelanosomal pH regulates Pmel17 amyloid formation and its subsequent dissolution in vivo.

Glycoprotein nonmetastatic melanoma protein b, a melanocytic cell marker, is a melanosome-specific and proteolytically released protein

Melanosomes are organelles specialized for the production of melanin pigment and are specifically produced by melanocytic cells. More than 150 pigmentation-related genes have been identified, including glycoprotein nonmetastatic melanoma protein b (GPNMB). A recent proteomics analysis revealed that GPNMB is localized in melanosomes, and GPNMB is a membrane-bound glycoprotein that shows high homology with a well-known melanosomal structural protein, Pmel17/gp100. In this study, we show that GPNMB is expressed in melanocytes of normal human skin, as well as in human melanoma cells. GPNMB is heavily glycosylated and is enriched in mature (stage III and IV) melanosomes in contrast to MART-1 and Pmel17, which are abundant in early (stage I and II) melanosomes. MART-1 and Pmel17 play critical roles in the maturation of early melanosomes; thus, we speculate that GPNMB might be important in the functions of late melanosomes, possibly their transport and/or transfer to keratinocytes. We also demonstrate that a secreted form of GPNMB is released by ectodomain shedding from the largely Golgi-modified form of GPNMB and that the PKC and Ca(2+) intracellular signaling pathways regulate that shedding. We conclude that GPNMB is a melanosomal protein that is released by proteolytic ectodomain shedding and might be a useful and specific histological marker of melanocytic cells.

N-terminal domains elicit formation of functional Pmel17 amyloid fibrils

Pmel17 is a transmembrane protein that mediates the early steps in the formation of melanosomes, the subcellular organelles of melanocytes in which melanin pigments are synthesized and stored. In melanosome precursor organelles, proteolytic fragments of Pmel17 form insoluble, amyloid-like fibrils upon which melanins are deposited during melanosome maturation. The mechanism(s) by which Pmel17 becomes competent to form amyloid are not fully understood. To better understand how amyloid formation is regulated, we have defined the domains within Pmel17 that promote fibril formation in vitro. Using purified recombinant fragments of Pmel17, we show that two regions, an N-terminal domain of unknown structure and a downstream domain with homology to a polycystic kidney disease-1 repeat, efficiently form amyloid in vitro. Analyses of fibrils formed in melanocytes confirm that the polycystic kidney disease-1 domain forms at least part of the physiological amyloid core. Interestingly, this same domain is also required for the intracellular trafficking of Pmel17 to multivesicular compartments within which fibrils begin to form. Although a domain of imperfect repeats (RPT) is required for fibril formation in vivo and is a component of fibrils in melanosomes, RPT is not necessary for fibril formation in vitro and in isolation is unable to adopt an amyloid fold in a physiologically relevant time frame. These data define the structural core of Pmel17 amyloid, imply that the RPT domain plays a regulatory role in timing amyloid conversion, and suggest that fibril formation might be physically linked with multivesicular body sorting.

The secreted form of a melanocyte membrane-bound glycoprotein (Pmel17/gp100) is released by ectodomain shedding

Ectodomain shedding is a proteolytic mechanism by which a transmembrane protein is converted into a secreted form. Pmel17/gp100 is a melanocyte-specific membrane-bound glycoprotein that has amyloid characteristics and forms fibrillar structures in melanosomes after a complex sequence of post-translational processing and trafficking events, including cleavage by a furin-like proprotein convertase (PC). A secreted form of Pmel17 (termed sPmel17) was also thought to be released due to cleavage by a PC. We used multidisciplinary approaches to demonstrate that sPmel17 is released by ectodomain shedding at the juxtamembrane and/or intramembrane motif and to show that this is independent of cleavage by a PC. We further show that sPmel17 consists of 2 fragments linked by disulfide bonds and that the shedding is inhibited at low temperature but not by metalloproteinase inhibitors. Moreover, treatment with a phorbol ester or a calmodulin inhibitor induces Pmel17 shedding. We also refine the reactivity of HMB50 and NKI/beteb, 2 monoclonal antibodies commonly used as melanoma-specific markers. The fact that those antibodies require physically separated domains of Pmel17 sheds interesting light on its 3-dimensional conformation. We conclude that sPmel17 is released by regulated proteolytic ectodomain shedding.-Hoashi, T., Tamaki, K., Hearing, V. J. The secreted form of a melanocyte membrane-bound glycoprotein (Pmel17/gp100) is released by ectodomain shedding.

Influence of melanosome dynamics on melanoma drug sensitivity

BACKGROUND

Malignant melanomas are intrinsically resistant to many conventional treatments, such as radiation and chemotherapy, for reasons that are poorly understood. Here we propose and test a model that explains drug resistance or sensitivity in terms of melanosome dynamics.

METHODS

The growth and sensitivity to cisplatin of MNT-1 cells, which are melanotic and enriched with mature stage III and IV melanosomes, and SK-MEL-28 cells, which have only immature stage I and II melanosomes, were compared using clonogenic assays. Differences in pigmentation, melanosome stages, melanosome number, and cellular structures in different cell lines in response to various treatments were examined by electron microscopy. The relative numbers of melanosomes of different stages were compared after treatment with 1-phenyl-2-thiourea. The relationship between drug transporter function and endogenous melanogenic toxicity was assessed by treating cells with the cyclosporin analog PSC-833 and by assessing vacuole formation and cell growth inhibition. All statistical tests were two-sided.

RESULTS

Endogenous melanogenic cytotoxicity, produced by damaged melanosomes, resulted in pronounced cell growth inhibition in MNT-1 cells compared with amelanotic SK-MEL-28 cells. The sensitivity to CDDP of MNT-1 cells was 3.8-fold higher than that of SK-MEL-28 cells (mean IC(50) for SK-MEL-28 and MNT-1 = 2.13 microM and 0.56 microM, respectively; difference = 1.57 microM, 95% confidence interval = 1.45 to 1.69; P = .0017). After treatment with 6.7 microM CDDP for 72 hours, the number of stage II-III melanosomes in surviving MNT-1 cells was 6.8-fold that of untreated cells. Modulation of MNT-1 cells to earlier-stage (II, II-III, III) melanosomes by treatment with the tyrosinase inhibitor 1-phenyl-2-thiourea dramatically increased CDDP resistance. Furthermore, PSC-833 principally suppressed MNT-1 melanotic cell growth via an elevation of autophagosome-like vacuolar structures, possibly by inhibiting melanosome membrane transporters.

CONCLUSIONS

Melanosome dynamics (including their biogenesis, density, status, and structural integrity) regulate the drug resistance of melanoma cells. Manipulation of melanosome functions may be an effective way to enhance the therapeutic activity of anticancer drugs against melanoma.

The repeat domain of the melanosome fibril protein Pmel17 forms the amyloid core promoting melanin synthesis

Pmel17 is a melanocyte protein necessary for eumelanin deposition 1 in mammals and found in melanosomes in a filamentous form. The luminal part of human Pmel17 includes a region (RPT) with 10 copies of a partial repeat sequence, pt.e.gttp.qv., known to be essential in vivo for filament formation. We show that this RPT region readily forms amyloid in vitro, but only under the mildly acidic conditions typical of the lysosome-like melanosome lumen, and the filaments quickly become soluble at neutral pH. Under the same mildly acidic conditions, the Pmel filaments promote eumelanin formation. Electron diffraction, circular dichroism, and solid-state NMR studies of Pmel17 filaments show that the structure is rich in beta sheet. We suggest that RPT is the amyloid core domain of the Pmel17 filaments so critical for melanin formation.

Physiological factors that regulate skin pigmentation

More than 150 genes have been identified that affect skin color either directly or indirectly, and we review current understanding of physiological factors that regulate skin pigmentation. We focus on melanosome biogenesis, transport and transfer, melanogenic regulators in melanocytes, and factors derived from keratinocytes, fibroblasts, endothelial cells, hormones, inflammatory cells, and nerves. Enzymatic components of melanosomes include tyrosinase, tyrosinase-related protein 1, and dopachrome tautomerase, which depend on the functions of OA1, P, MATP, ATP7A, and BLOC-1 to synthesize eumelanins and pheomelanins. The main structural component of melanosomes is Pmel17/gp100/Silv, whose sorting involves adaptor protein 1A (AP1A), AP1B, AP2, and spectrin, as well as a chaperone-like component, MART-1. During their maturation, melanosomes move from the perinuclear area toward the plasma membrane. Microtubules, dynein, kinesin, actin filaments, Rab27a, melanophilin, myosin Va, and Slp2-a are involved in melanosome transport. Foxn1 and p53 up-regulate skin pigmentation via bFGF and POMC derivatives including alpha-MSH and ACTH, respectively. Other critical factors that affect skin pigmentation include MC1R, CREB, ASP, MITF, PAX3, SOX9/10, LEF-1/TCF, PAR-2, DKK1, SCF, HGF, GM-CSF, endothelin-1, prostaglandins, leukotrienes, thromboxanes, neurotrophins, and neuropeptides. UV radiation up-regulates most factors that increase melanogenesis. Further studies will elucidate the currently unknown functions of many other pigment genes/proteins. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Genome-wide linkage scan localizes the harlequin locus in the Great Dane to chromosome 9

Harlequin is a coat pattern of the Great Dane characterized by ragged patches of full color on a white background. Harlequin patterning is a bigenic trait, resulting from the interaction of the merle allele of SILV, and a dominant modifier locus, H. Breeding data suggest that H is embryonic recessive lethal and that all harlequins are Hh. To identify linkage with the harlequin phenotype, 46 Great Danes from 5 pedigrees were genotyped for 280 microsatellite markers in a whole genome screen. One marker on the telomeric end of chromosome 9 was suggestive of linkage. Fine mapping of this region using additional microsatellite markers and 10 Great Danes from a sixth pedigree resulted in significant LOD scores for 2 markers. Reported herein is linkage mapping of the H locus to a 3.27 Mb region of chromosome 9 containing approximately 20 genes.

SLC24A5 encodes a trans-Golgi network protein with potassium-dependent sodium-calcium exchange activity that regulates human epidermal melanogenesis

A non-synonymous single nucleotide polymorphism in the human SLC24A5 gene is associated with natural human skin color variation. Multiple sequence alignments predict that this gene encodes a member of the potassium-dependent sodium-calcium exchanger family denoted NCKX5. In cultured human epidermal melanocytes we show using affinity-purified antisera that native human NCKX5 runs as a triplet of approximately 43 kDa on SDS-PAGE and is partially localized to the trans-Golgi network. Removal of the NCKX5 protein through small interfering RNA-mediated knockdown disrupts melanogenesis in human and murine melanocytes, causing a significant reduction in melanin pigment production. Using a heterologous expression system, we confirm for the first time that NCKX5 possesses the predicted exchanger activity. Site-directed mutagenesis of NCKX5 and NCKX2 in this system reveals that the non-synonymous single nucleotide polymorphism in SLC24A5 alters a residue that is important for NCKX5 and NCKX2 activity. We suggest that NCKX5 directly regulates human epidermal melanogenesis and natural skin color through its intracellular potassium-dependent exchanger activity.

Premelanosome amyloid-like fibrils are composed of only golgi-processed forms of Pmel17 that have been proteolytically processed in endosomes

Melanin pigments are synthesized within specialized organelles called melanosomes and polymerize on intraluminal fibrils that form within melanosome precursors. The fibrils consist of proteolytic fragments derived from Pmel17, a pigment cell-specific integral membrane protein. The intracellular pathways by which Pmel17 accesses melanosome precursors and the identity of the Pmel17 derivatives within fibrillar melanosomes have been a matter of debate. We show here that antibodies that detect Pmel17 within fibrillar melanosomes recognize only the luminal products of proprotein convertase cleavage and not the remaining products linked to the transmembrane domain. Moreover, antibodies to the N and C termini detect only Pmel17 isoforms present in early biosynthetic compartments, which constitute a large fraction of detectable steady state Pmel17 in cell lysates because of slow early biosynthetic transport and rapid consumption by fibril formation. Using an antibody to a luminal epitope that is destroyed upon modification by O-linked oligosaccharides, we show that all post-endoplasmic reticulum Pmel17 isoforms are modified by Golgi-associated oligosaccharide transferases, and that only processed forms contribute to melanosome biogenesis. These data indicate that Pmel17 follows a single biosynthetic route from the endoplasmic reticulum through the Golgi complex and endosomes to melanosomes, and that only fragments encompassing previously described functional luminal determinants are present within the fibrils. These data have important implications for the site and mechanism of fibril formation.

Melanosomes--dark organelles enlighten endosomal membrane transport

Melanosomes are tissue-specific lysosome-related organelles of pigment cells in which melanins are synthesized and stored. Analyses of the trafficking and fate of melanosomal components are beginning to reveal how melanosomes are formed through novel pathways from early endosomal intermediates. These studies unveil generalized structural and functional modifications of the endosomal system in specialized cells, and provide unexpected insights into the biogenesis of multivesicular bodies and how compartmentalization regulates protein refolding. Moreover, genetic disorders that affect the biogenesis of melanosomes and other lysosome-related organelles have shed light onto the molecular machinery that controls specialized endosomal sorting events.

Involvement of dynein and spectrin with early melanosome transport and melanosomal protein trafficking

Melanosomes are unique membrane-bound organelles specialized for the synthesis and distribution of melanin. Mechanisms involved in the trafficking of proteins to melanosomes and in the transport of mature pigmented melanosomes to the dendrites of melanocytic cells are being characterized, but details about those processes during early stages of melanosome maturation are not well understood. Early melanosomes must remain in the perinuclear area until critical components are assembled. In this study, we characterized the processing of two distinct melanosomal proteins, tyrosinase (TYR) and Pmel17, to elucidate protein processing in early or late steps of the secretory pathway, respectively, and to determine mechanisms underlying the subcellular localization and transport of early melanosomes. We used immunological, biochemical, and molecular approaches to demonstrate that the movement of early melanosomes in the perinuclear area depends primarily on microtubules but not on actin filaments. In contrast, the trafficking of TYR and Pmel17 depends on cytoplasmic dynein and its interaction with the spectrin/ankyrin system, which is involved with the sorting of cargo from the plasma membrane. These results provide important clues toward understanding the processes involved with early events in melanosome formation and transport.