Nanobeacons for Tracking Cells

Research paper p. 410--414
News & Views p. 383

Nanobeacons for tracking cells

Thinking small can sometimes be advantageous. Ralph Weissleder and colleagues have designed tiny, nanometer-sized magnetic particles that slip unnoticed into cells where they can serve as beacons for tracking the progress and movement of cells throughout the body. Using these particles, it should be possible to learn how cells migrate through the body and contribute to processes such as immune responses, the growth of blood vessels, and embryonic development.

A technique called magnetic resonance imaging (MRI), commonly used to map activity in the brain, provides a window both for following processes that occur deep within the body at the tissue or cellular level. For cellular tracking, however, current MRI methods require magnetic beads to be attached to the cell of interest, which results in their rapid recognition as foreign particles and subsequent elimination from the circulation.

To get around this, Weissleder and his collaborators have used tiny ~45 nm magnetic particles derivatized with an unusual protein, termed Tat, from the human immunodeficiency virus (HIV). This protein has the extraordinary property of chaperoning molecules from the outside to the inside of the cell by a mechanism as yet unknown. Reasoning that Tat-coated nanoparticles would be efficiently internalized by cells, Weissleder's team attached a triple magnetic, fluorescent, and isotopic label to Tat, and showed that the labeled, Tat-coated nanoparticles were efficiently taken up by both hematopoietic and neural progenitor cells. After injection into mice, the magnetic hematopoietic cells homed to bone marrow, and once the bone marrow was harvested, the cells could be detected by MRI at single cell resolution and then recaptured with magnetic separation columns.

The authors state that the technique should facilitate the detailed analysis of specific stem cell and organ interactions critical for the therapeutic use of stem cells.

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