Figure 1.

Strategies for conferring visibility in musculoskeletal molecular imaging. Bioluminescence imaging is frequently based on the Firefly luciferase (FLuc) reporter gene. Alternative luciferases used for bioluminescence imaging are Renilla or Gaussia luciferase, which catalyze the production of CO₂ and light (Em ~480 nm) in a cofactor-independent manner from coelenterazine and its derivatives instead of D-luciferin. The bacterial lux operon provides a basis for bioluminescence imaging without the need for exogenous substrate. Other reporter genes are the green fluorescent protein (GFP) for fluorescence imaging as well as the nuclear imaging reporter gene Herpes simplex virus 1 thymidine kinase (HSV1 TK), which relies on the preferential phosphorylation and hence cellular trapping of HSV1 thymidine kinase substrates. Employing probes represent a second important strategy for conferring imaging visibility. Two different types of probes can be used: static and activatable. Static probes that have been modified with a fluorophore can be imaged following their in-vivo concentration at a target site. Based on the observation that light absorption through biological tissues decreases significantly at wavelengths longer than approximately 600 nm, reaching a minimum around 750 nm [91], near-infrared (NIR, approximately 700 –900 nm) fluorophores are preferred optical labels for fluorescence imaging. Nuclear imaging of isotope-labeled static probes is also an established method. Popular isotopes for imaging applications include the gamma-emitters technetium-99m and indium-111 or the positron emitters fluorine-18 and copper-64. In contrast to static probes, activatable probes gain fluorescence through enzyme-mediated release of active fluorophores from a quenched precursor. Musculoskeletal molecular imaging studies have utilized a probe with a relatively broad selectivity to proteinases, including cathepsin B, L, S, and plasmin [32,33] or a probe preferentially cleaved by cathepsin K [10]. Utilizing antibodies instead of probes is an established strategy and includes fluorophore-labeled antibodies for fluorescence imaging and anti bodies that carry an isotope and thus are suitable for nuclear imaging. Particles composed of a cell membrane-penetrating shell and a paramagnetic iron-oxide core are used for cell labeling and subsequent detection on conventional proton MRI. Abbreviations: Ex, excitation wavelength; Em, emission wavelength; PET, positron emission tomography; γC/ SPECT, gamma camera/single photon emission computed tomography.