Dual loss of ER export and endocytic signals with altered melanosome morphology in the silver mutation of Pmel17

Protein Quality Control of NKCC2 in Bartter Syndrome and Blood Pressure Regulation

Mutations in NKCC2 generate antenatal Bartter syndrome type 1 (type 1 BS), a life-threatening salt-losing nephropathy characterized by arterial hypotension, as well as electrolyte abnormalities. In contrast to the genetic inactivation of NKCC2, inappropriate increased NKCC2 activity has been associated with salt-sensitive hypertension. Given the importance of NKCC2 in salt-sensitive hypertension and the pathophysiology of prenatal BS, studying the molecular regulation of this Na-K-2Cl cotransporter has attracted great interest. Therefore, several studies have addressed various aspects of NKCC2 regulation, such as phosphorylation and post-Golgi trafficking. However, the regulation of this cotransporter at the pre-Golgi level remained unknown for years. Similar to several transmembrane proteins, export from the ER appears to be the rate-limiting step in the cotransporter's maturation and trafficking to the plasma membrane. The most compelling evidence comes from patients with type 5 BS, the most severe form of prenatal BS, in whom NKCC2 is not detectable in the apical membrane of thick ascending limb (TAL) cells due to ER retention and ER-associated degradation (ERAD) mechanisms. In addition, type 1 BS is one of the diseases linked to ERAD pathways. In recent years, several molecular determinants of NKCC2 export from the ER and protein quality control have been identified. The aim of this review is therefore to summarize recent data regarding the protein quality control of NKCC2 and to discuss their potential implications in BS and blood pressure regulation.

Melanocytes in regenerative medicine applications and disease modeling

Melanocytes are dendritic cells localized in skin, eyes, hair follicles, ears, heart and central nervous system. They are characterized by the presence of melanosomes enriched in melanin which are responsible for skin, eye and hair pigmentation. They also have different functions in photoprotection, immunity and sound perception. Melanocyte dysfunction can cause pigmentary disorders, hearing and vision impairments or increased cancer susceptibility. This review focuses on the role of melanocytes in homeostasis and disease, before discussing their potential in regenerative medicine applications, such as for disease modeling, drug testing or therapy development using stem cell technologies, tissue engineering and extracellular vesicles.

Genetic Basis of Pigment Dispersion Syndrome and Pigmentary Glaucoma: An Update and Functional Insights

Pigment Dispersion Syndrome (PDS) and Pigmentary Glaucoma (PG) comprise a spectrum of ocular disorders characterized by iris pigment dispersion and trabecular meshwork changes, resulting in increased intraocular pressure and potential glaucomatous optic neuropathy. This review summarizes recent progress in PDS/PG genetics including rare pathogenic protein coding alterations (PMEL) and susceptibility loci identified from genome-wide association studies (GSAP and GRM5/TYR). Areas for future research are also identified, especially the development of efficient model systems. While substantial strides have been made in understanding the genetics of PDS/PG, our review identifies key gaps and outlines the future directions necessary for further advancing this important field of ocular genetics.

Biogenesis of fibrils requires C-mannosylation of PMEL

Premelanosome protein (PMEL), a melanocyte-specific glycoprotein, has an essential role in melanosome maturation, assembling amyloid fibrils for melanin deposition. PMEL undergoes several post-translational modifications, including N- and O-glycosylations, which are associated with proper melanosome development. C-mannosylation is a rare type of protein glycosylation at a tryptophan residue that might regulate the secretion and localization of proteins. PMEL has one putative C-mannosylation site in its core amyloid fragment (CAF); however, there is no report focusing on C-mannosylation of PMEL. To investigate this, we expressed recombinant PMEL in SK-MEL-28 human melanoma cells and purified the protein. Mass spectrometry analyses demonstrated that human PMEL is C-mannosylated at multiple tryptophan residues in its CAF and N-terminal fragment (NTF). In addition to the W153 or W156 residue (CAF), which lies in the consensus sequence for C-mannosylation, the W104 residue (NTF) was C-mannosylated without the consensus sequence. To determine the effects of the modifications, we deleted the PMEL gene by using CRISPR/Cas9 technology and re-expressed wild-type or C-mannosylation-defective mutants of PMEL, in which the C-mannosylated tryptophan was replaced with a phenylalanine residue (WF mutation), in SK-MEL-28 cells. Importantly, fibril-containing melanosomes were significantly decreased in W104F mutant PMEL-re-expressing cells compared with wild-type PMEL, observed using transmission electron microscopy. Furthermore, western blot and immunofluorescence analysis suggested that the W104F mutation may cause mild endoplasmic reticulumretention, possibly associated with early misfolding, and lysosomal misaggregation, thus reducing functional fibril formation. Our results demonstrate that C-mannosylation of PMEL is required for proper melanosome development by regulating PMEL-derived fibril formation.

Coordinated microRNA/mRNA Expression Profiles Reveal Unique Skin Color Regulatory Mechanisms in Chinese Giant Salamander (Andrias davidianus)

The Chinese giant salamander (Andrias davidianus) has been increasingly popular in the aquaculture market in China in recent years. In the breeding process of Andrias davidianus, we found that some albino individuals were extremely rare and could not be inherited stably, which severely limits their commercialization in the aquaculture market. In this study, we performed transcriptome and small RNA (sRNA) sequencing analyses in the skin samples of wild-type (WT) and albino (AL) Andrias davidianus. In total, among 5517 differentially expressed genes (DEGs), 2911 DEGs were down-regulated in AL, including almost all the key genes involved in melanin formation. A total of 25 miRNAs were differentially expressed in AL compared to WT, of which 17 were up-regulated. Through the integrated analysis, no intersection was found between the target genes of the differentially expressed miRNAs and the key genes for melanin formation. Gene Ontology (GO) and KEGG pathway analyses on DEGs showed that these genes involved multiple processes relevant to melanin synthesis and the key signal pathway MAPK. Interestingly, the transcription factors SOX10 and PAX3 and the Wnt signaling pathway that play a key role in other species were not included, while the other two transcription factors in the SOX family, SOX21 and SOX7, were included. After analyzing the key genes for melanin formation, it was interesting to note an alternative splicing form of the MITF in WT and a critical mutation of the SLC24A5 gene in AL, which might be the main reason for the skin color change of Andrias davidianus. The results contributed to understanding the molecular mechanism of skin pigmentation in Andrias davidianus and accelerating the acquisition process of individuals with specific body colors by genetic means.

A Comprehensive Review of Mammalian Pigmentation: Paving the Way for Innovative Hair Colour-Changing Cosmetics

The natural colour of hair shafts is formed at the bulb of hair follicles, and it is coupled to the hair growth cycle. Three critical processes must happen for efficient pigmentation: (1) melanosome biogenesis in neural crest-derived melanocytes, (2) the biochemical synthesis of melanins (melanogenesis) inside melanosomes, and (3) the transfer of melanin granules to surrounding pre-cortical keratinocytes for their incorporation into nascent hair fibres. All these steps are under complex genetic control. The array of natural hair colour shades are ascribed to polymorphisms in several pigmentary genes. A myriad of factors acting via autocrine, paracrine, and endocrine mechanisms also contributes for hair colour diversity. Given the enormous social and cosmetic importance attributed to hair colour, hair dyeing is today a common practice. Nonetheless, the adverse effects of the long-term usage of such cosmetic procedures demand the development of new methods for colour change. In this context, case reports of hair lightening, darkening and repigmentation as a side-effect of the therapeutic usage of many drugs substantiate the possibility to tune hair colour by interfering with the biology of follicular pigmentary units. By scrutinizing mammalian pigmentation, this review pinpoints key targetable processes for the development of innovative cosmetics that can safely change the hair colour from the inside out.

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.

Melanogenesis and the Targeted Therapy of Melanoma

Pigment production is a unique character of melanocytes. Numerous factors are linked with melanin production, including genetics, ultraviolet radiation (UVR) and inflammation. Understanding the mechanism of melanogenesis is crucial to identify new preventive and therapeutic strategies in the treatment of melanoma. Here, we reviewed the current available literatures on the mechanisms of melanogenesis, including the signaling pathways of UVR-induced pigment production, MC1R's central determinant roles and MITF as a master transcriptional regulator in melanogenesis. Moreover, we further highlighted the role of targeting BRAF, NRAS and MC1R in melanoma prevention and treatment. The combination therapeutics of immunotherapy and targeted kinase inhibitors are becoming the newest therapeutic option in advanced melanoma.

Diacidic Motifs in the Carboxyl Terminus Are Required for ER Exit and Translocation to the Plasma Membrane of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome (BS1), a life-threatening kidney disease. We have previously demonstrated that the BS1 variant Y998X, which deprives NKCC2 from its highly conserved dileucine-like motifs, compromises co-transporter surface delivery through ER retention mechanisms. However, whether these hydrophobic motifs are sufficient for anterograde trafficking of NKCC2 remains to be determined. Interestingly, sequence analysis of NKCC2 C-terminus revealed the presence of consensus di-acidic (D/E-X-D/E) motifs, ⁹⁴⁹EEE⁹⁵¹ and ¹⁰¹⁹DAELE¹⁰²³, located upstream and downstream of BS1 mutation Y998X, respectively. Di-acidic codes are involved in ER export of proteins through interaction with COPII budding machinery. Importantly, whereas mutating ⁹⁴⁹EEE⁹⁵¹ motif to ⁹⁴⁹AEA⁹⁵¹ had no effect on NKCC2 processing, mutating ¹⁰¹⁹DAE¹⁰²¹ to ¹⁰¹⁹AAA¹⁰²¹ heavily impaired complex-glycosylation and cell surface expression of the cotransporter in HEK293 and OKP cells. Most importantly, triple mutation of D, E and E residues of ¹⁰¹⁹DAELE¹⁰²³ to ¹⁰¹⁹AAALA¹⁰²³ almost completely abolished NKCC2 complex-glycosylation, suggesting that this mutant failed to exit the ER. Cycloheximide chase analysis demonstrated that the absence of the terminally glycosylated form of ¹⁰¹⁹AAALA¹⁰²³ was caused by defects in NKCC2 maturation. Accordingly, co-immunolocalization experiments revealed that ¹⁰¹⁹AAALA¹⁰²³ was trapped in the ER. Finally, overexpression of a dominant negative mutant of Sar1-GTPase abolished NKCC2 maturation and cell surface expression, clearly indicating that NKCC2 export from the ER is COPII-dependent. Hence, our data indicate that in addition to the di-leucine like motifs, NKCC2 uses di-acidic exit codes for export from the ER through the COPII-dependent pathway. We propose that any naturally occurring mutation of NKCC2 interfering with this pathway could form the molecular basis of BS1.

Attenuation Effect of Radiofrequency Irradiation on UV-B-Induced Skin Pigmentation by Decreasing Melanin Synthesis and through Upregulation of Heat Shock Protein 70

Excess melanin deposition in the skin causes cosmetic problems. HSP70 upregulation decreases microphthalmia-associated transcription factor (MITF) expression, which eventually decreases tyrosinase activity and melanogenesis. Ultraviolet (UV) radiation upregulates p53, which increases the melanocortin receptor (MC1R) and MITF. Furthermore, HSP70 decreases p53 and radiofrequency irradiation (RF) increases HSP70. We evaluated whether RF increased HSP70 and decreased p53, consequently decreasing the MITF/tyrosinase pathway and melanogenesis in UV-B radiated animal skin. Various RF combinations with 50, 100, and 150 ms and 5, 10, and 15 W were performed on the UV-B radiated mouse skin every 2 d for 28 d. When RF was performed with 100 ms/10 W, melanin deposition, evaluated by Fontana–Masson staining, decreased without skin crust formation in the UV-B radiated skin. Thus, we evaluated the effect of RF on decreasing melanogenesis in the HEMn and UV-B radiated skin at a setting of 100 ms/10 W. HSP70 expression was decreased in the UV-B radiated skin but was increased by RF. The expression of p53, MC1R, and MITF increased in the UV-B radiated skin but was decreased by RF. The expression of p53, MC1R, and MITF increased in the α-MSH treated HEMn but was decreased by RF. The decreasing effects of RF on p53, MC1R, CREB and MITF were higher than those of HSP70-overexpressed HEMn. The decreasing effect of RF on p53, MC1R, CREB, and MITF disappeared in the HSP70-silenced HEMn. MC1R, CREB, and MITF were not significantly decreased by the p53 inhibitor in α-MSH treated HEMn. RF induced a greater decrease in MC1R, CREB, and MITF than the p53 inhibitor. Therefore, RF may have decreased melanin synthesis by increasing HSP70 and decreasing p53, thus decreasing MC1R/CREB/MITF and tyrosinase activity.

The impaired unfolded protein-premelanosome protein and transient receptor potential channels-autophagy axes in apoptotic melanocytes in vitiligo

Vitiligo is an autoimmune skin disease, characterized by depigmentation and epidermal melanocytes loss. The specific mechanisms underlying vitiligo have not been fully understood. As a result, treating vitiligo is a dermatological challenge. Recently, much attention has been paid to the dysfunction and interaction of organelles under environmental stress. The impaired organelles could generate misfolded proteins, particularly accumulated toxic premelanosome protein (PMEL) amyloid oligomers, activating the autoimmune system and cause melanocyte damage. Unfolded protein response (UPR) dysfunction accelerates toxic PMEL accumulation. Herein, we presented a narrative review on UPR's role in vitiligo, the misfolded PMEL-induced attack of the autoimmune system under autophagy dysfunction caused by abnormal activation of transient receptor potential (TRP) channels and the background of UPR system defects in melanocytes. All of these mechanisms were integrated to form UPR/PMEL-TRP channels/autophagy axis, providing a new understanding of vitiligo pathogenesis.

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.

Melanosome biogenesis in the pigmentation of mammalian skin

Melanins, the main pigments of the skin and hair in mammals, are synthesized within membrane-bound organelles of melanocytes called melanosomes. Melanosome structure and function are determined by a cohort of resident transmembrane proteins, many of which are expressed only in pigment cells, that localize specifically to melanosomes. Defects in the genes that encode melanosome-specific proteins or components of the machinery required for their transport in and out of melanosomes underlie various forms of ocular or oculocutaneous albinism, characterized by hypopigmentation of the hair, skin and eyes and by visual impairment. We review major components of melanosomes, including the enzymes that catalyze steps in melanin synthesis from tyrosine precursors, solute transporters that allow these enzymes to function, and structural proteins that underlie melanosome shape and melanin deposition. We then review the molecular mechanisms by which these components are biosynthetically delivered to newly forming melanosomes-many of which are shared by other cell types that generate cell type-specific lysosome-related organelles. We also highlight unanswered questions that need to be addressed by future investigation.

Biogenesis, Membrane Trafficking, Functions, and Next Generation Nanotherapeutics Medicine of Extracellular Vesicles

Extracellular vesicles (EVs) are a heterogeneous group of membrane-limited vesicles and multi-signal messengers loaded with biomolecules. Exosomes and ectosomes are two different types of EVs generated by all cell types. Their formation depends on local microdomains assembled in endocytic membranes for exosomes and in the plasma membrane for ectosomes. Further, EV release is a fundamental process required for intercellular communication in both normal physiology and pathological conditions to transmit/exchange bioactive molecules to recipient cells and the extracellular environment. The unique structure and composition of EVs enable them to serve as natural nanocarriers, and their physicochemical properties and biological functions can be used to develop next-generation nano and precision medicine. Knowledge of the cellular processes that govern EVs biology and membrane trafficking is essential for their clinical applications. However, in this rapidly expanding field, much remains unknown regarding EV origin, biogenesis, cargo sorting, and secretion, as well as EV-based theranostic platform generation. Hence, we present a comprehensive overview of the recent advances in biogenesis, membrane trafficking, and functions of EVs, highlighting the impact of nanoparticles and oxidative stress on EVs biogenesis and release and finally emphasizing the role of EVs as nanotherapeutic agents.

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.

Molecular and Biochemical Basis of Minocycline-Induced Hyperpigmentation-The Study on Normal Human Melanocytes Exposed to UVA and UVB Radiation

Minocycline is a drug which induces skin hyperpigmentation. Its frequency reaches up to 50% of treated patients. The adverse effect diminishes the great therapeutic potential of minocycline, including antibacterial, neuroprotective, anti-inflammatory and anti-cancer actions. It is supposed that an elevated melanin level and drug accumulation in melanin-containing cells are related to skin hyperpigmentation. This study aimed to evaluate molecular and biochemical mechanism of minocycline-induced hyperpigmentation in human normal melanocytes, as well as the contribution of UV radiation to this side effect. The experiments involved the evaluation of cyto- and phototoxic potential of the drug using cell imaging with light and confocal microscopes as well as biochemical and molecular analysis of melanogenesis. We showed that minocycline induced melanin synthesis in epidermal melanocytes. The action was intensified by UV irradiation, especially with the UVB spectrum. Minocycline stimulated the expression of microphthalmia-associated transcription factor (MITF) and tyrosinase (TYR) gene. Higher levels of melanin and increased activity of tyrosinase were also observed in treated cells. Moreover, minocycline triggered the supranuclear accumulation of tyrosinase, similar to UV radiation. The decreased level of premelanosome protein PMEL17 observed in all minocycline-treated cultures suggests disorder of the formation, maturation or distribution of melanosomes. The study revealed that minocycline itself was able to enhance melanin synthesis. The action was intensified by irradiation, especially with the UVB spectrum. Demonstrated results confirmed the potential role of melanin and UV radiation minocycline-induced skin hyperpigmentation.

Melanosome transport and regulation in development and disease

Melanosomes are specialized membrane-bound organelles that synthesize and organize melanin, ultimately providing color to the skin, hair, and eyes. Disorders in melanogenesis and melanosome transport are linked to pigmentary diseases, such as Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, and Griscelli syndrome. Clinical cases of these pigmentary diseases shed light on the molecular mechanisms that control melanosome-related pathways. However, only an improved understanding of melanogenesis and melanosome transport will further the development of diagnostic and therapeutic approaches. Herein, we review the current literature surrounding melanosomes with particular emphasis on melanosome membrane transport and cytoskeleton-mediated melanosome transport. We also provide perspectives on melanosome regulatory mechanisms which include hormonal action, inflammation, autophagy, and organelle interactions.

Research Techniques Made Simple: Cell Biology Methods for the Analysis of Pigmentation

Pigmentation of the skin and hair represents the result of melanin biosynthesis within melanosomes of epidermal melanocytes, followed by the transfer of mature melanin granules to adjacent keratinocytes within the basal layer of the epidermis. Natural variation in these processes produces the diversity of skin and hair color among human populations, and defects in these processes lead to diseases such as oculocutaneous albinism. While genetic regulators of pigmentation have been well studied in human and animal models, we are still learning much about the cell biological features that regulate melanogenesis, melanosome maturation, and melanosome motility in melanocytes, and have barely scratched the surface in our understanding of melanin transfer from melanocytes to keratinocytes. Herein, we describe cultured cell model systems and common assays that have been used by investigators to dissect these features and that will hopefully lead to additional advances in the future.

Elucidation of Melanogenesis Cascade for Identifying Pathophysiology and Therapeutic Approach of Pigmentary Disorders and Melanoma

Melanogenesis is the biological and biochemical process of melanin and melanosome biosynthesis. Melanin is formed by enzymic reactions of tyrosinase family proteins that convert tyrosine to form brown-black eumelanin and yellow-red pheomelanin within melanosomal compartments in melanocytes, following the cascades of events interacting with a series of autocrine and paracrine signals. Fully melanized melanosomes are delivered to keratinocytes of the skin and hair. The symbiotic relation of a melanocyte and an associated pool of keratinocytes is called epidermal melanin unit (EMU). Microphthalmia-associated transcription factor (MITF) plays a vital role in melanocyte development and differentiation. MITF regulates expression of numerous pigmentation genes for promoting melanocyte differentiation, as well as fundamental genes for maintaining cell homeostasis. Diseases involving alterations of EMU show various forms of pigmentation phenotypes. This review introduces four major topics of melanogenesis cascade that include (1) melanocyte development and differentiation, (2) melanogenesis and intracellular trafficking for melanosome biosynthesis, (3) melanin pigmentation and pigment-type switching, and (4) development of a novel therapeutic approach for malignant melanoma by elucidation of melanogenesis cascade.

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.

Genetics of Equine Ocular Disease

Horses perform in a variety of disciplines that are visually demanding, and any disease impacting the eye has the potential to threaten vision and thus the utility of the horse. Advances in equine genetics have enabled the understanding of some inherited ocular disorders and ocular manifestations and are enabling cross-species comparisons. Genetic testing for multiple congenital ocular anomalies, congenital stationary night blindness, equine recurrent uveitis, and squamous cell carcinoma can identify horses with or at risk for disease and thus can assist in clinical management and breeding decisions. This article describes the current knowledge of inherited ocular disorders.

The role of exosomes in metastasis and progression of melanoma

The mechanisms of melanoma metastasis have been the subject of extensive research for decades. Improved diagnostic and therapeutic strategies are of increasing importance for the treatment of melanoma due to its high burden of mortality in the advanced stages of the disease. Intercellular communication is a critical event for the progression of cancer. Collective evidence suggests that exosomes, small extracellular membrane vesicles released by the cells, are important facilitators of intercellular communication between the cells and the surrounding environment. Although the emerging field of exosomes is rapidly gaining traction in the scientific community, there is limited knowledge regarding the role of exosomes in melanoma. This review discusses the multifaceted role of melanoma-derived exosomes in promoting the process of metastasis by modulating the invasive and angiogenic capacity of malignant cells. The future implications of exosome research and the therapeutic potential of exosomes are also discussed.

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.

The PIKfyve complex regulates the early melanosome homeostasis required for physiological amyloid formation

The metabolism of PI(3,5)P2 is regulated by the PIKfyve, VAC14 and FIG4 complex, mutations in which are associated with hypopigmentation in mice. These pigmentation defects indicate a key, but as yet unexplored, physiological relevance of this complex in the biogenesis of melanosomes. Here, we show that PIKfyve activity regulates formation of amyloid matrix composed of PMEL protein within the early endosomes in melanocytes, called stage I melanosomes. PIKfyve activity controls the membrane remodeling of stage I melanosomes, which regulates PMEL abundance, sorting and processing. PIKfyve activity also affects stage I melanosome kiss-and-run interactions with lysosomes, which are required for PMEL amyloidogenesis and the establishment of melanosome identity. Mechanistically, PIKfyve activity promotes both the formation of membrane tubules from stage I melanosomes and their release by modulating endosomal actin branching. Taken together, our data indicate that PIKfyve activity is a key regulator of the melanosomal import-export machinery that fine tunes the formation of functional amyloid fibrils in melanosomes and the maintenance of melanosome identity.This article has an associated First Person interview with the first author of the paper.

Intrinsic and extrinsic regulation of human skin melanogenesis and pigmentation

In human skin, melanogenesis is a tightly regulated process. Indeed, several extracellular signals are transduced via dedicated signalling pathways and mostly converge to MITF, a transcription factor integrating upstream signalling and regulating downstream genes involved in the various inherent mechanisms modulating melanogenesis. The synthesis of melanin pigments occurs in melanocytes inside melanosomes where melanogenic enzymes (tyrosinase and related proteins) are addressed with the help of specific protein complexes. The melanosomes loaded with melanin are then transferred to keratinocytes. A more elaborate level of melanogenesis regulation comes into play via the action of non-coding RNAs (microRNAs, lncRNAs). Besides this canonical regulation, melanogenesis can also be modulated by other non-specific intrinsic pathways (hormonal environment, inflammation) and by extrinsic factors (solar irradiation such as ultraviolet irradiation, environmental pollution). We developed a bioinformatic interaction network gathering the multiple aspects of melanogenesis and skin pigmentation as a resource to better understand and study skin pigmentation biology.

Molecular Genetics of Pigment Dispersion Syndrome and Pigmentary Glaucoma: New Insights into Mechanisms

We explore the ideas and advances surrounding the genetic basis of pigment dispersion syndrome (PDS) and pigmentary glaucoma (PG). As PG is the leading cause of nontraumatic blindness in young adults and current tailored interventions have proven ineffective, a better understanding of the underlying causes of PDS, PG, and their relationship is essential. Despite PDS being a subclinical disease, a large proportion of patients progress to PG with associated vision loss. Decades of research have supported a genetic component both for PDS and conversion to PG. We review the body of evidence supporting a genetic basis in humans and animal models and reevaluate classical mechanisms of PDS/PG considering this new evidence.

Ocular Albinism Type 1 Regulates Melanogenesis in Mouse Melanocytes

To investigate whether ocular albinism type 1 (OA1) is differentially expressed in the skin of mice with different coat colors and to determine its correlation with coat color establishment in mouse, the expression patterns and tissue distribution characterization of OA1 in the skin of mice with different coat colors were qualitatively and quantitatively analyzed by real-time quantitative PCR (qRT-PCR), immunofluorescence staining and Western blot. The qRT-PCR analysis revealed that OA1 mRNA was expressed in all mice skin samples tested, with the highest expression level in brown skin, a moderate expression level in black skin and the lowest expression level in gray skin. Positive OA1 protein bands were also detected in all skin samples by Western blot analysis. The relative expression levels of OA1 protein in both black and brown skin were significantly higher than that in gray skin, but there was no significant difference between black and brown mice. Immunofluorescence assays revealed that OA1 was mainly expressed in the hair follicle matrix, the inner and outer root sheath in the skin tissues with different coat colors. To get further insight into the important role of OA1 in the melanocytes’ pigmentation, we transfected the OA1 into mouse melanocytes and then detected the relative expression levels of pigmentation-related gene. Simultaneously, we tested the melanin content of melanocytes. As a result, the overexpression of OA1 significantly increased the expression levels of microphthalmia-associated transcription factor (MITF), tyrosinase (TYR), tyrosinase-related protein 1 (TRP1) and premelanosome protein (PMEL). However, the tyrosinase-related protein 2 (TRP2) level was attenuated. By contrast, the level of glycoprotein non-metastatic melanoma protein b (GPNMB) was unaffected by OA1 overexpression. Furthermore, we observed a significant increase in melanin content in mouse melanocyte transfected OA1. Therefore, we propose that OA1 may participate in the formation of coat color by regulating the level of MITF and the number, size, motility and maturation of melanosome.

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.

Impaired Lysosomal Integral Membrane Protein 2-dependent Peroxiredoxin 6 Delivery to Lamellar Bodies Accounts for Altered Alveolar Phospholipid Content in Adaptor Protein-3-deficient pearl Mice

The Hermansky Pudlak syndromes (HPS) constitute a family of disorders characterized by oculocutaneous albinism and bleeding diathesis, often associated with lethal lung fibrosis. HPS results from mutations in genes of membrane trafficking complexes that facilitate delivery of cargo to lysosome-related organelles. Among the affected lysosome-related organelles are lamellar bodies (LB) within alveolar type 2 cells (AT2) in which surfactant components are assembled, modified, and stored. AT2 from HPS patients and mouse models of HPS exhibit enlarged LB with increased phospholipid content, but the mechanism underlying these defects is unknown. We now show that AT2 in the pearl mouse model of HPS type 2 lacking the adaptor protein 3 complex (AP-3) fails to accumulate the soluble enzyme peroxiredoxin 6 (PRDX6) in LB. This defect reflects impaired AP-3-dependent trafficking of PRDX6 to LB, because pearl mouse AT2 cells harbor a normal total PRDX6 content. AP-3-dependent targeting of PRDX6 to LB requires the transmembrane protein LIMP-2/SCARB2, a known AP-3-dependent cargo protein that functions as a carrier for lysosomal proteins in other cell types. Depletion of LB PRDX6 in AP-3- or LIMP-2/SCARB2-deficient mice correlates with phospholipid accumulation in lamellar bodies and with defective intraluminal degradation of LB disaturated phosphatidylcholine. Furthermore, AP-3-dependent LB targeting is facilitated by protein/protein interaction between LIMP-2/SCARB2 and PRDX6 in vitro and in vivo Our data provide the first evidence for an AP-3-dependent cargo protein required for the maturation of LB in AT2 and suggest that the loss of PRDX6 activity contributes to the pathogenic changes in LB phospholipid homeostasis found HPS2 patients.

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.

BLOC-2 targets recycling endosomal tubules to melanosomes for cargo delivery

Quantitative analyses of melanosome cargo localization and trafficking and of endosomal membrane dynamics in immortalized melanocytes from mouse Hermansky–Pudlak syndrome models show that BLOC-2 functions to specify the delivery of recycling endosomal cargo transport intermediates to maturing melanosomes.

Hermansky–Pudlak syndrome (HPS) is a group of disorders characterized by the malformation of lysosome-related organelles, such as pigment cell melanosomes. Three of nine characterized HPS subtypes result from mutations in subunits of BLOC-2, a protein complex with no known molecular function. In this paper, we exploit melanocytes from mouse HPS models to place BLOC-2 within a cargo transport pathway from recycling endosomal domains to maturing melanosomes. In BLOC-2–deficient melanocytes, the melanosomal protein TYRP1 was largely depleted from pigment granules and underwent accelerated recycling from endosomes to the plasma membrane and to the Golgi. By live-cell imaging, recycling endosomal tubules of wild-type melanocytes made frequent and prolonged contacts with maturing melanosomes; in contrast, tubules from BLOC-2–deficient cells were shorter in length and made fewer, more transient contacts with melanosomes. These results support a model in which BLOC-2 functions to direct recycling endosomal tubular transport intermediates to maturing melanosomes and thereby promote cargo delivery and optimal pigmentation.

Iron upregulates melanogenesis in cultured retinal pigment epithelial cells

The purpose of our studies was to examine the relationship between iron and melanogenesis in retinal pigment epithelial cells, as prior observations had suggested that iron may promote melanogenesis. This relationship has potential clinical importance, as both iron overload and hyperpigmentation are associated with age-related macular degeneration (AMD). Human fetal retinal pigment epithelial cells and ARPE-19 cells were treated with iron in the form of ferric ammonium citrate, after which quantitative RT-PCR and electron microscopy were performed. Melanogenesis genes tyrosinase, tyrosinase-related protein 1, Hermansky-Pudlak Syndrome 3, premelanosome protein and dopachrome tautomerase were upregulated, as was the melanogenesis-controlling transcription factor, microphthalmia-associated transcription factor (MITF). Iron-treated cells had increased pigmentation and melanosome number. Multiple transcription factors upstream of MITF were upregulated by iron.

Function, therapeutic potential and cell biology of BACE proteases: current status and future prospects

The β-site APP cleaving enzymes 1 and 2 (BACE1 and BACE2) were initially identified as transmembrane aspartyl proteases cleaving the amyloid precursor protein (APP). BACE1 is a major drug target for Alzheimer's disease because BACE1-mediated cleavage of APP is the first step in the generation of the pathogenic amyloid-β peptides. BACE1, which is highly expressed in the nervous system, is also required for myelination by cleaving neuregulin 1. Several recent proteomic and in vivo studies using BACE1- and BACE2-deficient mice demonstrate a much wider range of physiological substrates and functions for both proteases within and outside of the nervous system. For BACE1 this includes axon guidance, neurogenesis, muscle spindle formation, and neuronal network functions, whereas BACE2 was shown to be involved in pigmentation and pancreatic β-cell function. This review highlights the recent progress in understanding cell biology, substrates, and functions of BACE proteases and discusses the therapeutic options and potential mechanism-based liabilities, in particular for BACE inhibitors in Alzheimer's disease. The protease BACE1 is a major drug target in Alzheimer disease. Together with its homolog BACE2, both proteases have an increasing number of functions within and outside of the nervous system. This review highlights recent progress in understanding cell biology, substrates, and functions of BACE proteases and discusses the therapeutic options and potential mechanism-based liabilities, in particular for BACE inhibitors in Alzheimer disease.

Hypopigmentation in Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) is characterized by oculocutaneous albinism, bleeding tendency, and ceroid deposition which often leads to death in midlife. Currently, nine genes have been identified as causative for HPS in humans. Hypopigmentation is the prominent feature of HPS, attributable to the disrupted biogenesis of melanosome, a member of the lysosome-related organelle (LRO) family. Current understanding of the cargo transporting mechanisms into the melanosomes expands our knowledge of the pathogenesis of hypopigmentation in HPS patients.

Melanosome-autonomous regulation of size and number: the OA1 receptor sustains PMEL expression

Little is known as to how cells ensure that organelle size and number are coordinated to correctly couple organelle biogenesis to the demands of proliferation or differentiation. OA1 is a melanosome-associated G-protein-coupled receptor involved in melanosome biogenesis during melanocyte differentiation. Cells lacking OA1 contain fewer, but larger, mature melanosomes. Here, we show that OA1 loss of function reduces both the basal expression and the α-melanocyte-stimulating hormone/cAMP-dependent induction of the microphthalmia-associated transcription factor (MITF), the master regulator of melanocyte differentiation. In turn, this leads to a significant reduction in expression of PMEL, a major melanosomal structural protein, but does not affect tyrosinase and melanin levels. In line with its pivotal role in sensing melanosome maturation, OA1 expression rescues melanosome biogenesis, activates MITF expression and thereby coordinates melanosome size and number, providing a quality control mechanism for the organelle in which resides. Thus, resident sensor receptors can activate a transcriptional cascade to specifically promote organelle biogenesis.

The prion-like domain in the exomer-dependent cargo Pin2 serves as a trans-Golgi retention motif

Prion and prion-like domains (PLDs) are found in many proteins throughout the animal kingdom. We found that the PLD in the S. cerevisiae exomer-dependent cargo protein Pin2 is involved in the regulation of protein transport and localization. The domain serves as a Pin2 retention signal in the trans-Golgi network (TGN). Pin2 is localized in a polarized fashion at the plasma membrane of the bud early in the cell cycle and the bud neck at cytokinesis. This polarized localization is dependent on both exo- and endocytosis. Upon environmental stress, Pin2 is rapidly endocytosed, and the PLD aggregates and causes sequestration of Pin2. The aggregation of Pin2 is reversible upon stress removal and Pin2 is rapidly re-exported to the plasma membrane. Altogether, these data uncover a role for PLDs as protein localization elements.

The B7-1 cytoplasmic tail enhances intracellular transport and mammalian cell surface display of chimeric proteins in the absence of a linear ER export motif

Membrane-tethered proteins (mammalian surface display) are increasingly being used for novel therapeutic and biotechnology applications. Maximizing surface expression of chimeric proteins on mammalian cells is important for these applications. We show that the cytoplasmic domain from the B7-1 antigen, a commonly used element for mammalian surface display, can enhance the intracellular transport and surface display of chimeric proteins in a Sar1 and Rab1 dependent fashion. However, mutational, alanine scanning and deletion analysis demonstrate the absence of linear ER export motifs in the B7 cytoplasmic domain. Rather, efficient intracellular transport correlated with the presence of predicted secondary structure in the cytoplasmic tail. Examination of the cytoplasmic domains of 984 human and 782 mouse type I transmembrane proteins revealed that many previously identified ER export motifs are rarely found in the cytoplasmic tail of type I transmembrane proteins. Our results suggest that efficient intracellular transport of B7 chimeric proteins is associated with the structure rather than to the presence of a linear ER export motif in the cytoplasmic tail, and indicate that short (less than ~ 10-20 amino acids) and unstructured cytoplasmic tails should be avoided to express high levels of chimeric proteins on mammalian cells.

BACE2 processes PMEL to form the melanosome amyloid matrix in pigment cells

Amyloids are often associated with pathologic processes such as in Alzheimer's disease (AD), but can also underlie physiological processes such as pigmentation. Formation of pathological and functional amyloidogenic substrates can require precursor processing by proteases, as exemplified by the generation of Aβ peptide from amyloid precursor protein (APP) by beta-site APP cleaving enzyme (BACE)1 and γ-secretase. Proteolytic processing of the pigment cell-specific Melanocyte Protein (PMEL) is also required to form functional amyloid fibrils during melanogenesis, but the enzymes involved are incompletely characterized. Here we show that the BACE1 homologue BACE2 processes PMEL to generate functional amyloids. BACE2 is highly expressed in pigment cells and Bace2(-/-) but not Bace1(-/-) mice display coat color defects, implying a specific role for BACE2 during melanogenesis. By using biochemical and morphological analyses, combined with RNA silencing, pharmacologic inhibition, and BACE2 overexpression in a human melanocytic cell line, we show that BACE2 cleaves the integral membrane form of PMEL within the juxtamembrane domain, releasing the PMEL luminal domain into endosomal precursors for the formation of amyloid fibrils and downstream melanosome morphogenesis. These studies identify an amyloidogenic substrate of BACE2, reveal an important physiological role for BACE2 in pigmentation, and highlight analogies in the generation of PMEL-derived functional amyloids and APP-derived pathological amyloids.

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.

Cell type-specific Rab32 and Rab38 cooperate with the ubiquitous lysosome biogenesis machinery to synthesize specialized lysosome-related organelles

Lysosome-related organelles (LROs) exist in specialized cells to serve specific functions and typically co-exist with conventional lysosomes. The biogenesis of LROs is known to utilize much of the common protein machinery used in the transport of integral membrane proteins to lysosomes. Consequently, an outstanding question in the field has been how specific cargoes are trafficked to LROs instead of lysosomes, particularly in cells that simultaneously produce both organelles. One LRO, the melanosome, is responsible for the production of the pigment melanin and has long been used as a model system to study the formation of specialized LROs. Importantly, melanocytes, where melanosomes are synthesized, are a cell type that also produces lysosomes and must therefore segregate traffic to each organelle. Two small GTPases, Rab32 and Rab38, are key proteins in the biogenesis of melanosomes and were recently shown to redirect the ubiquitous machinery—BLOC-2, AP-1 and AP-3—to traffic specialized cargoes to melanosomes in melanocytes. In addition, the study revealed Rab32 and Rab38 have both redundant and unique roles in the trafficking of melanin-producing enzymes and overall melanosome biogenesis. Here we review these findings, integrate them with previous knowledge on melanosome biogenesis and discuss their implications for biogenesis of other LROs.

Multiple evolutionarily conserved Di-leucine like motifs in the carboxyl terminus control the anterograde trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the (1081)LLV(1083) motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, (1038)LL(1039) and (1048)LI(1049), that are required for ER exit and surface expression of the co-transporter. Double mutations of (1038)LL(1039) or (1048)LI(1049) to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that (1038)AA(1039) and (1048)AA(1049) were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.

Multiple evolutionarily conserved Di-leucine like motifs in the carboxyl terminus control the anterograde trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the (1081)LLV(1083) motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, (1038)LL(1039) and (1048)LI(1049), that are required for ER exit and surface expression of the co-transporter. Double mutations of (1038)LL(1039) or (1048)LI(1049) to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that (1038)AA(1039) and (1048)AA(1049) were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.

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.

Differential recognition of a dileucine-based sorting signal by AP-1 and AP-3 reveals a requirement for both BLOC-1 and AP-3 in delivery of OCA2 to melanosomes

OCA2 is used as a model melanosome cargo protein to define primary sequence elements required for acidic dileucine–motif binding to adaptors AP-1 and AP-3. OCA2 must bind to AP-3 for melanosome localization. BLOC-1 is also required and thus can cooperate with either adaptor for cargo delivery to lysosome-related organelles.

Cell types that generate unique lysosome-related organelles (LROs), such as melanosomes in melanocytes, populate nascent LROs with cargoes that are diverted from endosomes. Cargo sorting toward melanosomes correlates with binding via cytoplasmically exposed sorting signals to either heterotetrameric adaptor AP-1 or AP-3. Some cargoes bind both adaptors, but the relative contribution of each adaptor to cargo recognition and their functional interactions with other effectors during transport to melanosomes are not clear. Here we exploit targeted mutagenesis of the acidic dileucine–based sorting signal in the pigment cell–specific protein OCA2 to dissect the relative roles of AP-1 and AP-3 in transport to melanosomes. We show that binding to AP-1 or AP-3 depends on the primary sequence of the signal and not its position within the cytoplasmic domain. Mutants that preferentially bound either AP-1 or AP-3 each trafficked toward melanosomes and functionally complemented OCA2 deficiency, but AP-3 binding was necessary for steady-state melanosome localization. Unlike tyrosinase, which also engages AP-3 for optimal melanosomal delivery, both AP-1– and AP-3–favoring OCA2 variants required BLOC-1 for melanosomal transport. These data provide evidence for distinct roles of AP-1 and AP-3 in OCA2 transport to melanosomes and indicate that BLOC-1 can cooperate with either adaptor during cargo sorting to LROs.