Fig. 6: Concept of imaging senescent cells with a β-gal sensitive Gd-chelate.

A Concept of β-Gal-activation by cleavage of a pendant carboxylate ligand. The β-Gal-activated MR probe is composed of gadolinium (Gd III) conjugated to a hydrophilic β-Gal substrate. Before enzyme reaction, the coordination sphere of Gd is saturated by a β-galactosyl moiety and a bridging ligand, preventing the interaction with water protons and resulting in low signal on T1-weighted MRI scans. After exposure to β-gal, the β-galactosyl moiety is cleaved, opening the coordination site of Gd III for proton interaction and resulting in an increased (hyperintense) MRI signal on T₁-weighted MR scans. B Molecular changes of β-gal sensitive Gd-chelate after exposure to β-gal. Upon stress/damage, cells in the joint compartment undergo senescence, resulting in the expression of β-gal. Before activation by β-gal, the β-galactosyl moiety (blue) of the Gd-chelate prohibits water access to Gd (III). After exposure to β-gal, the β-galactosyl moiety is cleaved and an electron cascade (red) provides an open coordination site for water to bind to Gd (III). C Study design. Viable or senescent mesenchymal stromal cells were implanted in cartilage defects of the distal femur of pig knee joints. The β-gal responsive Gd-chelate was injected into the knee joint, leading to activation of the β-gal sensitive Gd-chelate in senescent cell transplants and not viable cell transplants, as measured by MRI.