Melanins and Melanosomes

Biosynthesis, Structure, Physiological and Pathological Functions

Jan Borovansky, Patrick A. Riley, Patrick A. Riley, Jan Borovansky, Patrick A. Riley, Jan Borovansky

• Publisher: Wiley-Blackwell
• Year: 2011
• Print ISBN:
  9783527328925
• Edition: 1
• eBook ISBN:
  9783527644551
• Topic: Biological Sciences
• Subtopic: Cell Biology
• Index: Biological Sciences

The surface pigmentation of vertebrates is controlled by specialized cells able to synthesize a variety of pigments collectively known as melanins. Recent research has shown that melanins are produced not only in the skin but also in many other sites such as the eye, inner ear, muscles, etc., - where they are engaged in some unanticipated roles. The details of the synthetic pathway, the complexities of its regulation and biological significance that have been unravelled in recent research comprise a fascinating story and are of key importance in understanding the nature of diseases, including malignant melanoma one of the most rapidly spreading cancers.

1

History of Melanosome Research

Jan Borovanský

1.1 Introduction

Melanosomes were first proposed as specific organelles, unique to pigment cells, in a preliminary publication that appeared on 30 July 1960 [1]. An announcement had been made at the 21st Annual Meeting of the Society for Investigative Dermatology, at Miami Beach, Florida, USA on 13 June 1960 [2] and the news, that the chemical composition and enzyme activities in melanosomes and mitochondria are completely different, was considered to be of such significance that it appeared in a newspaper report (Figure 1.1). Similar data, with an emphasis on terminology, were published in 1963 [3].

Figure 1.1 Announcement of the independent status of melanosome in Medical News on 5 July 1960.

This advance was the result of collaborative work between M. Seiji (1926–1982), at that time working at the Department of Dermatology, Harvard Medical School in Boston under the leadership of T.B. Fitzpatrick (1919–2003) (Figure 1.2), and H. Blaschko and M.S.C. Birbeck, with whom Dr Fitzpatrick established scientific cooperation during his tenure of a Commonwealth Fellowship at the Department of Biochemistry, Radcliffe Infirmary in Oxford.

Figure 1.2 Professor Makoto Seiji (left) and Professor Thomas B. Fitzpatrick (right) in 1972.

The history of melanosome research can be formally divided into three parts: (i) the pre-Seiji era (prior to 1960), (i) the Seiji era (1960–1982), and (iii) the post-Seiji era (1983–).

1.2 Melanosome Research in the Pre-Seiji Era

The first description of mammalian pigment cells was published by Gustav Simon in 1841 [4] who observed round and stellate pigment cells in the hair bulbs of pig embryos. It was preceeded in 1838 by Purkyn

’s description of pigment in the cells of the substantia nigra, which not only drew attention to pigment granules, but also noted the rise in their numbers with age [5]. We have to admire these early reports because their authors, armed only with primitive light microscopes, were able to ascertain that melanin was not diffusely distributed in the cytoplasm of pigmented cells, but was present in the form of discrete aggregates [5, 6] (Figures 1.3 and 1.4).

Figure 1.3 “Chromatophore” from donkey conjuctiva [7].

Figure 1.4 Cells of substantia nigra containing neuromelanin [5].

Deciphering the old literature is problematical as authors often fail to distinguish between melanin (the pigment itself), melanoprotein (the natural melanin–protein complex), and melanin granules (the subcellular organelle). If the method of separation is not adequately described, it is difficult to be certain what material was studied and any conclusions can be misleading [8]. The lack of electron microscopic identification of isolated material led to many misinterpretations; for example, the “melanopseudoglobulin” studied by Greenstein et al. [9] was later shown to be melanosomes [10] and Bolt’s “melanoprotein” [11], widely used in biophysical studies, turned out to consist of damaged melanosomes [12]. Mason et al. [10] posed the question of whether melanin granules were particles with a specific structure or consisted of random aggregates of precipitated metabolic products. The introduction of electron microscopy was able to resolve this matter and Laxer et al. [13] were able to discern an inner ultrastructure in isolated melanosomes. The first clear pictures were obtained only in 1956 [14].

An avalanche of papers in subsequent years brought with it enormous amounts of information on the ultrastructure of melanosomes and its changes during melanosome development (good examples are [15–17]). Other papers (reviewed in [18]) brought together ultrastructural and biochemical data that, in combination, laid the basis for the nomenclature of melanosomal ontogenesis.

By comparison with the morphological data, biochemical investigations of melanosomes were more modest, mainly due to the fact that ultrastructural data were derived from studies of intact cells or tissues, whereas biochemical research used samples prepared by relatively harsh preparative procedures. These samples sometimes consisted of melanins, or altered melanosomes, or their fragments, usually without any check of their nature or homogeneity [18].

The aim of researchers in the nineteenth century was not to prepare subcellular particles or native melanoproteins, but to separate the colored pigment (“Farbstoff” = melanin in the terminology of that time). The presence of protein in the isolated material was considered an unwanted contaminant [19]. Probably the first mild separation protocol was used by J.J. Berzelius [20]. He investigated pigment (melanosomes?) obtained from eye membranes by water extraction, and noticed its insolubility in acids and limited solubility in alkali. Similar mild extraction procedures were used by Landolt [21] and Mörner [22]. The early isolation procedures were reviewed by Waelsch [23]. He studied “natural melanin” from human melanoma metastases and horse choroids, confirmed the presence of protein attached to pigment, and suggested that melanin could be synthesized from the cyclic amino acids present in the protein moiety; this idea has not been abandoned till now. Herrmann and Boss [24] demonstrated dopa oxidase activity in the fraction of melanin granules from ciliary bodies of cattle eyes, but, as their samples were contaminated with mitochondria, they demonstrated the presence of mitochondrial enzyme markers as well. In 1949, du Buy et al. concluded that melanosomes are modified mitochondria typical of pigment cells [25]. It is interesting that du Buy [26] and other authors [27] did not abandon the mitochondrial theory of melanosome origin even in 1963 (i.e., 2 years after the formulation of Seiji’s melanosomal concept) and even published their papers in the same volume in which Seiji et al. published detailed confirmation of their model [28].

It is interesting that history has disregarded the contribution of Stein [29] who, several years before the work of Seiji et al., using a separation procedure of his own, isolated melanin granules from ox choroids and analyzed their content not only of melanin, but also lipids, carbohydrates, RNA, and metals (including the pioneer finding of a high level of zinc), and concluded that the chemical composition of melanin granules is completely different from mitochondria.

The ability of melanin in melanin granules, isolated from Harding-Passey melanoma and from the ink sac of Loligo opalescens, to act as a cation exchanger [30], and the demonstration of free radical activity in melanin-containing tissues [31] also rank among the observations of the pre-Seiji era.

1.3 Melanosome Research in the Seiji Era

1.3.1 Terminology of Melanosomes

The demonstration of melanosomes as unique pigment cell organelles possessing developmental stages prompted the introduction of a system of terminology that reflected the characteristics of the various states. Until 1961 the common term for all varieties of these organelles was melanin (or pigment) granule [1, 2]. The first system of nomenclature [2] described three stages in the ontogenesis of melanosomes:

i) Premelanosomes: spherical organelles.

ii) Melanosomes: organelles with an internal structure and tyrosinase activity.

iii) Melanin granules: melanoprotein polymer.

A second terminological system was proposed [3, 26] consisting of three developmental stages plus a final product. Thus:

• Stage I (first stage): biosynthesis of protein.
• Stage II (intermediate stage): biosynthesis of organelle.
• Stage III (late phase): biosynthesis of melanin.
• Final product: melanin granule.

These nomenclature systems introduced a certain degree of confusion, particularly as the term melanin granule had been used to describe pigment granules at any developmental stage. In an attempt to establish a consensus, Fitzpatrick et al. [32, 33] circulated a postal questionnaire seeking opinions about the adequacy of the terms in common use in pigment cell research and, with the approval of the participants of the Sixth International Pigment Cell Conference in 1965 in Sofia, Bulgaria, recommended the use of two terms:

• Melanosome: a discrete melanin-containing organelle in which melanization is complete as indicated by its almost uniform density by electron microscopy and the absence of demonstrable tyrosinase activity.
• Premelanosome: a term applied to all the stages in melanosome biogenesis that precede the fully developed state. Within the restrictions of this general definition, the premelanosomal stage mig...

Table of contents

• Cover
• Related Titles
• Title page
• Copyright page
• Dedication
• Preface
• List of Contributors
• 1 History of Melanosome Research
• 2 Classical and Nonclassical Melanocytes in Vertebrates
• 3 Biological Chemistry of o-Quinones
• 4 Biosynthesis of Melanins
• 5 Inhibitors and Enhancers of Melanogenesis
• 6 Structure of Melanins
• 7 Properties and Functions of Ocular Melanins and Melanosomes
• 8 Biological Role of Neuromelanin in the Human Brain and Its Importance in Parkinson’s Disease
• 9 Biogenesis of Melanosomes
• 10 Transport and Distribution of Melanosomes
• 11 Genetics of Melanosome Structure and Function
• 12 Physiological and Pathological Functions of Melanosomes
• 13 Dysplastic Nevi as Precursor Melanoma Lesions
• Index