The photobiology of melanin

Melanin Nanoparticles Obtained from Preformed Recombinant Melanin by Bottom-Up and Top-Down Approaches

Melanin is an insoluble, amorphous polymer that forms planar sheets that aggregate naturally to create colloidal particles with several biological functions. Based on this, here, a preformed recombinant melanin (PRM) was utilized as the polymeric raw material to generate recombinant melanin nanoparticles (RMNPs). These nanoparticles were prepared using bottom-up (nanocrystallization-NC, and double emulsion-solvent evaporation-DE) and top-down (high-pressure homogenization-HP) manufacturing approaches. The particle size, Z-potential, identity, stability, morphology, and solid-state properties were evaluated. RMNP biocompatibility was determined in human embryogenic kidney (HEK293) and human epidermal keratinocyte (HEKn) cell lines. RMNPs prepared by NC reached a particle size of 245.9 ± 31.5 nm and a Z-potential of -20.2 ± 1.56 mV; 253.1 ± 30.6 nm and -39.2 ± 0.56 mV compared to that obtained by DE, as well as RMNPs of 302.2 ± 69.9 nm and -38.6 ± 2.25 mV using HP. Spherical and solid nanostructures in the bottom-up approaches were observed; however, they were an irregular shape with a wide size distribution when the HP method was applied. Infrared (IR) spectra showed no changes in the chemical structure of the melanin after the manufacturing process but did exhibit an amorphous crystal rearrangement according to calorimetric and PXRD analysis. All RMNPs presented long stability in an aqueous suspension and resistance to being sterilized by wet steam and ultraviolet (UV) radiation. Finally, cytotoxicity assays showed that RMNPs are safe up to 100 μg/mL. These findings open new possibilities for obtaining melanin nanoparticles with potential applications in drug delivery, tissue engineering, diagnosis, and sun protection, among others.

Effect of constitutional pigmentation on ultraviolet B-induced DNA damage in fair-skinned people

Ultraviolet light has been implicated as a dominant factor in skin cancer development. Skin pigmentation is traditionally regarded as an important protection against skin cancer. Yet, little is known about how skin pigmentation is modulating induction of DNA damage, which is the primary event in carcinogenesis. We applied a recently developed 32P-postlabeling technique to measure the effect of constitutional pigmentation on the formation of major ultraviolet-induced DNA damage in human skin in vivo. The induction of photoproducts showed a statistically significant negative correlation with erythemal response and skin pigmentation. Our results demonstrated that the constitutional pigmentation is efficiently guarding DNA against the formation of photoproducts. The difference in melanin content is likely to be one of the reasons for the observed interindividual variation in levels of DNA damage after the uniform exposure to ultraviolet B.

THE DARK SIDE OF THE MYCELIUM: Melanins of Phytopathogenic Fungi

Melanins are darkly pigmented polymers that protect organisms against environmental stress. Even when not directly involved in pathogenesis, fungal melanin is likely required by melanizing phytopathogens for survival in the environment. However, some phytopathogenic fungi that produce melanized appressoria for host invasion require appressorial melanogenesis for pathogenicity. Much less is known about the role melanins play in pathogenesis during infection by other phytopathogens that do not rely on appressoria for host penetration. Here we focus on one such phytopathogenic fungus, Gaeumannomyces graminis var. tritici, the etiologic agent of the devastating root disease of cereals, take-all. This fungus is lightly pigmented in culture, but requires melanin biosynthesis for pathogenesis, perhaps to produce melanized, ectotrophic macrohyphae on roots. However, the constitutively melanized, asexual Phialophora anamorph of G. graminis var. tritici is nonpathogenic. In addition, melanization of G. graminis var. graminis is not required to produce root disease on its rice host. Explanations for these apparent contradictions are suggested, as are other functions for the melanins of phytopathogenic fungi.