Inactivation of Pmel alters melanosome shape but has only a subtle effect on visible pigmentation

Area of Science:

• Cell Biology
• Genetics
• Biochemistry

Background:

• PMEL is an amyloidogenic protein crucial for melanosome biogenesis and eumelanin deposition in pigment cells.
• Allelic variants of PMEL in various species are linked to altered pigmentation, but their functional consequences (gain- or loss-of-function) remain unclear.
• A complete loss-of-function mutation for PMEL has not been previously reported in mammals.

Purpose of the Study:

• To investigate the function of PMEL by creating and characterizing a mouse model with a complete Pmel gene knockout (Pmel⁻/⁻).
• To determine the role of PMEL in melanosome morphology, eumelanin production, and overall pigmentation.
• To clarify the nature of previously observed PMEL allelic variants in vertebrates.

Main Methods:

• Generation of a Pmel knockout mouse line (Pmel⁻/⁻) through gene inactivation.
• Phenotypic analysis of Pmel⁻/⁻ mice, including viability, fertility, and developmental assessment.
• Microscopic examination of melanosome morphology in melanocytes from various tissues (skin, retinal pigment epithelium, uveal melanocytes).
• Assessment of coat color and eumelanin content in hair of Pmel⁻/⁻ mice, including crosses with other pigment mutation models (e.g., brown/Tyrp1).

Main Results:

• Pmel⁻/⁻ mice are viable, fertile, and exhibit no major developmental abnormalities.
• Melanosomes in Pmel⁻/⁻ melanocytes are spherical, unlike the oblong shape in wild-type mice.
• Inactivation of Pmel results in a mild coat color phenotype, most evident in combination with the brown/Tyrp1 mutation, suggesting other vertebrate alleles may be dominant-negative.
• Despite mild visible effects on coat color, Pmel inactivation causes a significant reduction in hair eumelanin content.

Conclusions:

• PMEL plays a critical role in establishing proper melanosome structure and is essential for efficient epidermal pigmentation.
• The Pmel knockout mouse model provides a valuable tool for studying PMEL function and the mechanisms underlying pigmentation.
• The findings suggest that PMEL is indispensable for maximizing eumelanin deposition in hair, highlighting its importance beyond structural roles in melanosomes.