Abstract
Melanosomes are highly specialized organelles that produce and store the pigment melanin, thereby fulfilling essential functions within their host organism. Besides having obvious cosmetic consequences – determining the color of skin, hair and the iris – they contribute to photochemical protection from ultraviolet radiation, as well as to vision (by defining how much light enters the eye). Though melanosomes can be beneficial for health, abnormalities in their structure can lead to adverse effects. Knowledge of their ultrastructure will be crucial to gaining insight into the mechanisms that ultimately lead to melanosome-related diseases. However, due to their small size and electron-dense content, physiologically intact melanosomes are recalcitrant to study by common imaging techniques such as light and transmission electron microscopy. In contrast, X-ray-based methodologies offer both high spatial resolution and powerful penetrating capabilities, and thus are well suited to study the ultrastructure of electron-dense organelles in their natural, hydrated form. Here, we report on the application of small-angle X-ray scattering – a method effective in determining the three-dimensional structures of biomolecules – to whole, hydrated murine melanosomes. The use of complementary information from the scattering signal of a large ensemble of suspended organelles and from single, vitrified specimens revealed a melanosomal sub-structure whose surface and bulk properties differ in two commonly used inbred strains of laboratory mice. Whereas melanosomes in C57BL/6J mice have a well-defined surface and are densely packed with 40-nm units, their counterparts in DBA/2J mice feature a rough surface, are more granular and consist of 60-nm building blocks. The fact that these strains have different coat colors and distinct susceptibilities to pigment-related eye disease suggest that these differences in size and packing are of biological significance.
Highlights
Melanosomes [1] are organelles that contain melanin [2] and fulfill important biochemical tasks in their host cells
Melanosomes have been the subject of numerous microscopy studies aiming to provide insights into their roles in health and disease [7,8] the exceptional electron density of melanin has been an obstacle in probing the ultrastructure of melanosomes by traditional approaches such as transmission electron microscopy (TEM)
Small angle X-ray scattering at ambient temperatures We began our characterization of melanosome features with
Summary
Melanosomes [1] are organelles that contain melanin [2] and fulfill important biochemical tasks in their host cells. They are responsible for the color of hair, skin and the iris, provide protection against the damaging effects of UV radiation and scavenge free radicals [3]. Melanosomes are increasingly being studied using specialized approaches such as 3D electron tomography [9] and STXM (scanning transmission X-ray microscopy) [10,11]. Data from each of these approaches have led to new insights into aspects of melanosome ultrastructure that had previously been impervious to enquiry
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