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Melanin In Bloom: My Journal

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Zhang, J. et al. Prophenoloxidase-mediated ex vivo immunity to delay fungal infection after insect ecdysis. Front. Immunol 8, 1445 (2017). González‐Santoyo, I. & Córdoba‐Aguilar, A. Phenoloxidase: a key component of the insect immune system. Entomol. Exp. Appl. 142, 1–16 (2012). Dubovskiy, I. M., Kryukova, N. A., Glupov, V. V. & Ratcliffe, N. A. Encapsulation and nodulation in insects. Invertebr. Survival J. 13, 229–246 (2016). Liu Y, Hong L, Kempf VR, Wakamatsu K, Ito S, Simon JD (June 2004). "Ion-exchange and adsorption of Fe(III) by Sepia melanin". Pigment Cell Research. 17 (3): 262–9. doi: 10.1111/j.1600-0749.2004.00140.x. PMID 15140071.

Melasma. People with melasma have brown or blue-gray patches on their faces or arms. This condition may be caused by hormones, exposure to the sun or birth control pills. Prescription creams, laser skin resurfacing or chemical peels can help lighten the dark patches. Sarangarajan R, Apte SP (2005). "Melanization and phagocytosis: implications for age related macular degeneration". Molecular Vision. 11: 482–90. PMID 16030499. Sarangarajan R, Apte SP (2005). "Melanin aggregation and polymerization: possible implications in age-related macular degeneration". Ophthalmic Research. 37 (3): 136–41. doi: 10.1159/000085533. PMID 15867475. S2CID 27499198. Shrestha, S. & Kim, Y. Oenocytoid cell lysis to release prophenoloxidase is induced by eicosanoid via protein kinase C. J. Asia Pac. Entomol. 12, 301–305 (2009).The second method used to evaluate the melanization response to C. neoformans was tissue clearing, which enabled us to visualize melanized nodules in situ deep within the larvae. We modified a recently developed protocol 40 to view the nodules that formed during infections, and saw the native structures of the nodules and their anatomical location in the larvae. This offers an advantage over dissection of uncleared larvae, because during the dissection process: (1) the tissue organization is disrupted, (2) some organs such as the nerve cord and cardiac system might be disrupted, and (3) the geography of infection patterns may not be apparent. Additionally, melanized nodules may not be visible within or behind opaque tissues and organs. Tissue dissection of opaque larvae was helpful when evaluating tissue tropism since tissue boundaries may not be fully visible in clarified larvae. A bias involved in studying fungal infections using both tissue clearing and dissection is that the non-melanized nodules or fungi may be missed, as unpigmented fungi will likely blend in with surrounding tissue. However, in the clearing method, we viewed the nodules throughout the entire depth of the larvae at a low to moderate (×4–40) magnification using light microscopy. However, the objective and microscope limitations only permitted imaging the superficial melanized nodules at ×100 magnification, which provided a lower resolution of the nodules compared to the imaging of the extracted hemolymph. While in the case of C. neoformans, the nodules within the hemolymph appeared congruent to those viewed in situ, that might not always be the case. Nodules in extracted hemolymph during other fungal infection may not be entirely representative of those found throughout the entire larvae, so only viewing the hemolymph nodules may give a biased understanding of the fungal infection. During cryptococcal infection of G. mellonella, melanin encapsulation of the fungus within nodules was associated with diminished or lost fluorescence signal in these GFP-expressing C. neoformans strains. Additionally, the melanin-encapsulated fungi that remained GFP-positive had weaker signals and the intensity of the GFP signal was more intense for the non-melanin-encapsulated fungi within the nodules. The expression of GFP in these cells is under the control of an actin promotor, and while actin is generally presumed to be constitutively expressed in cells, growth conditions have been shown to lead to some alterations in cryptococcal actin expression 49, 50. If the environmental conditions within the nodule abolished actin expression in some cells without killing the fungus, we would expect that condition to equally affect the melanin and non-melanin-encapsulated fungi, and as a result, see similar GFP-negative: GFP-positive ratios between the melanin-encapsulated and not melanin-encapsulated cells. The association between melanin encapsulation and disappearance in GFP fluorescence provides strong evidence for the notion that the melanization reaction kills fungal cells during infection. This is the first evidence, to the best of our understanding, that G. mellonella immune melanization directly and effectively neutralizes C. neoformans during infection and demonstrated that melanin encapsulation results in fungal death within the insect. Previously, the death of microbes, specifically bacteria, was attributed to the enzymatic activity of the melanin-producing phenoloxidase (PO) in an in vitro reaction 16. In addition to our association of melanin encapsulation and fungal death in vivo, we sought to reproduce these results in vitro using extracted hemolymph in buffer. We used the PO-specific inhibitor, phenylthiourea (PTU), we found that PO-inhibited wells of hemolymph had higher recoverable CFUs of C. neoformans compared to the uninhibited wells. The inverse correlation of melanization with CFUs further supports the claim that melanin plays a role in neutralizing C. neoformans. Since we only assayed CFUs from these in vitro experiments, we cannot determine whether the melanization in the in vitro experiments directly killed the fungus or just inhibited fungal growth.

Notably, we observed the formation of melanin-evasive pseudohyphae in C. auris when exposed to the G. mellonella hemolymph. These findings corroborate previous studies showing that C. auris forms pseudohyphae during G. mellonella infection 54, and indicate that the stress and cues from the hemolymph can induce morphological changes in multiple fungal species. Pseudohyphae and enlarged giant cells in C. auris have been previously linked to exposure to genotoxic stress, indicating that the melanization reaction can induce DNA damage 55. Following tissue clearing of C. auris infected larvae, we observed pseudohyphal structures in situ. We also observed melanin-encapsulated fungi within aggregates surrounding trachea, as previously reported 54, 56, although the frequency of this association was difficult to ascertain, with aggregates of melanin-encapsulated C. auris was also found unassociated with trachea, including in the insect’s head capsule. a b Solano, F. (2014). "Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes". New Journal of Science. 2014 (498276): 1–28. doi: 10.1155/2014/498276. Pheomelanins (or phaeomelanins) impart a range of yellowish to reddish colors. [10] Pheomelanins are particularly concentrated in the lips, nipples, glans of the penis, and vagina. [11] When a small amount of brown eumelanin in hair (which would otherwise cause blond hair) is mixed with red pheomelanin, the result is orange hair, which is typically called "red" or "ginger" hair. Pheomelanin is also present in the skin, and redheads consequently often have a more pinkish hue to their skin as well. Exposure of the skin to ultraviolet light increases pheomelanin content, as it does for eumelanin; but rather than absorbing light, pheomelanin within the hair and skin reflect yellow to red light, which may increase damage from UV radiation exposure. [12] Millar, S. E.; Miller, M. W.; Stevens, M. E.; Barsh, G. S. (October 1995). "Expression and transgenic studies of the mouse agouti gene provide insight into the mechanisms by which mammalian coat color patterns are generated". Development. 121 (10): 3223–3232. doi: 10.1242/dev.121.10.3223. PMID 7588057.

Cichorek, Mirosława; Wachulska, Małgorzata; Stasiewicz, Aneta; Tymińska, Agata (20 February 2013). "Skin melanocytes: biology and development". Advances in Dermatology and Allergology. 30 (1): 30–41. doi: 10.5114/pdia.2013.33376. PMC 3834696. PMID 24278043. Cable J, Huszar D, Jaenisch R, Steel KP (February 1994). "Effects of mutations at the W locus (c-kit) on inner ear pigmentation and function in the mouse". Pigment Cell Research. 7 (1): 17–32. doi: 10.1111/j.1600-0749.1994.tb00015.x. PMID 7521050.

As with peoples having migrated northward, those with light skin migrating toward the equator acclimatize to the much stronger solar radiation. Nature selects for less melanin when ultraviolet radiation is weak. Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of sun tanning. Dark-skinned people, who produce more skin-protecting eumelanin, have a greater protection against sunburn and the development of melanoma, a potentially deadly form of skin cancer, as well as other health problems related to exposure to strong solar radiation, including the photodegradation of certain vitamins such as riboflavins, carotenoids, tocopherol, and folate. [68] Eisenman, H. C., Duong, R., Chan, H., Tsue, R. & McClelland, E. E. Reduced virulence of melanized Cryptococcus neoformans in Galleria mellonella. Virulence 5, 611–618 (2014).Mark Heiden (8 July 2020). "New biomaterial could shield against harmful radiation". Northwestern University . Retrieved 29 January 2023. We also examined the melanization response to Candida albicans infection. C. albicans is known to trigger large-scale systemic melanization in G. mellonella larvae 39, 53. Like C. neoformans, we found melanized nodules in the hemolymph from larvae infected with C. albicans. Interestingly, the center of these nodules had melanized and smoothened areas that seemed more amorphous than those seen with C. neoformans, and additionally, we saw hyphal structures appeared less melanized than the spherical yeast-like structures. Using the tissue clarification method, we noted that the melanin-encapsulated C. albicans formed large rope-like aggregates without tissue tropism, with yeast being preferentially melanized over hyphal cells. Using in vitro time-lapse microscopy, we found that rapid melanization occurred, even in the absence of hemocytes. Additionally, after the melanization plateaus, the surviving fungus can break free from the melanin encapsulation and undergo melanin-evasive filamentation. This is followed by production of laterally budding blastoconidium and a bloom in melanization around these newly formed yeast cells. Similar fungal morphologies and timelines were observed in dissected infected larvae, although the temporal kinetics were less resolved and identification of blastoconidium was less clear. Together, these data paint an interesting picture and allow insight into the pathogenesis of C. albicans within G. mellonella host. Hence, it appears that the melanin encapsulation can clear most of the yeast upon infection; however, cells that survive can then filament and evade subsequent melanin-mediated killing. The hyphae are known to penetrate and infect organs within the insect 23. The hyphae then produce yeast, which again triggers a burst of melanization that would likely cause damage to the surrounding tissue and eventually death of the organism.

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