Near-IR fluorescent dye measures age of skin and blood vessels

The fluorescent dye could be useful to better understand the role of elastin in biological processes.

Content Dam Bow Online Articles 2018 04 Rsz Fig1

A team of researchers at the Center for Self-Assembly and Complexity within the Institute for Basic Science (IBS; Daejeon, Korea) has synthesized a contrast agent to observe and measure elastin, the protein that gives strength to blood vessel walls and flexibility to skin. The fluorescent dye could be useful to better understand the role of elastin in biological processes, and to verify the health of blood vessels and organs.

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"Since elastin is located in the subcutaneous tissue, it is difficult to visualize with current methods. Moreover, it is a complex protein to work with, as it has various forms, variable chemical linkages, and it is renownedly difficult to make a model for computational study. For the screening, we used intact elastin from throughout the whole body to avoid such problems," explains Chang Young-Tae, a corresponding author of the paper describing the work.

Content Dam Bow Online Articles 2018 04 Rsz Fig1
ElaNIR screening and chemical structure is shown, where ElaNIR is a member of a library of fluorescent dyes that can emit near-infrared rays (a); it is a neutral molecule with two positive and two negative chemical groups, guaranteeing that it does not bind aspecifically to other biological molecules. ElaNIR highlights elastic layers, known as lamellae, in a rabbit's aorta (b).

The research team developed a near-infrared (near-IR) fluorescent contrast agent for elastin, called ElaNIR (Elastin Near-InfraRed), that is capable of detecting the elasticity of tissues in living animals. The method is based on near-IR rays with wavelengths from 700 to 900 nm, which can reach elastin because of their excellent skin permeability. ElaNIR was selected from a collection of fluorescence molecules that emit near-IR rays. Generally, fluorescent materials tend to stick to other substances, resulting in a blurred image area around the target. On the other hand, ElaNIR has an equal number of positive and negative charges within the same molecule, which has been shown to minimize non-specific binding to serum proteins, normal tissues, and organs, which provides cleaner images.

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ElaNIR study of mouse organs with high elastin content is shown.

ElaNIR not only emits fluorescence at the near-IR range, but also allows visualization of elastin-containing structures via photoacoustic spectroscopy. In photoacoustic imaging, the absorbed light energy is converted into ultrasonic sound, which can penetrate skin better than fluorescence. As a result, photoacoustics can provide higher sensitivity and higher spatial resolution in ElaNIR compared to fluorescence imaging. Thanks to this technique, the scientists were able to acquire 3D tomographies by detecting ultrasonic waves generated by absorbed near-IR light.

The scientists demonstrated the selective binding of ElaNIR to elastin using the cross section of a rabbit’s aorta. Then, they applied it to study mouse organs with high elastin content. ElaNIR was able to highlight elastin in lungs, skin, urinary bladder, kidney, aorta, and ears.

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ElaNIR shows differences in elastin concentration and distribution between young and old mice, where one-month-old mice have higher concentrations of elastin on the skin than their 10-month-old counterparts.

Finally, ElaNIR was tested on living animals. When injected intravenously in old and young mice, the team was able to compare the amount of elastin in the skin. In the older rodents, the amount of elastin was significantly reduced (1.8X) compared to the younger ones. Using the photoacoustic technique, a high-resolution 3D tomographic image was obtained, which visually shows the distribution of elastin. Another promising feature of ElaNIR is its rapid removal from the system: more than 90% of the injected dose was cleared from the blood in four hours.

Full details of the work appear in the journal Chem.

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