MRI-Based Detection of Alkaline Phosphatase Gene Reporter Activity Using a Porphyrin Solubility Switch

Abstract

SUMMARY The ability to map patterns of gene expression noninvasively in living animals could have impact in many areas of biology. Reporter systems compatible with magnetic resonance imaging (MRI) could be particularly valuable, but existing strategies tend to lack sensitivity or specificity. Here we address the challenge of MRI-based gene mapping using the reporter enzyme secreted alkaline phosphatase (SEAP), in conjunction with a water soluble metalloporphyrin contrast agent. SEAP cleaves the porphyrin into an insoluble product that accumulates at sites of enzyme expression and can be visualized by MRI and optical absorbance. The contrast mechanism functions in vitro, in brain slices, and in animals. The system also provides the possibility of readout both in the living animal and by post mortem histology, and it notably does not require intracellular delivery of the contrast agent. The solubility switch mechanism used to detect SEAP could be adapted for imaging of additional reporter enzymes or endogenous targets.

Tumor‐specific expression and detection of a CEST reporter gene

“…Many reporters have been identified as candidate MR contrast‐generating genes. Those include: the β‐galactosidase‐Gd 3+ ‐containing galactopyranosyl ring reporter‐probe pair for T 1 contrast ; the tyrosinase‐paramagnetic iron pair for T 2 contrast ; the transferrin receptor‐monocrystalline iron oxide nanocompound pair for T 2 contrast ; the ferritin and endogenous iron pair for T 2 contrast ; the Mag A‐iron pair for T 2 contrast ; and, the secreted alkaline phosphatase (SEAP)‐phosphorylated metalloporphyrin pair for T 1 contrast . We have developed an artificial gene, the lysine‐rich protein (LRP), as a reporter for chemical exchange saturation transfer (CEST) MRI .…”

Activatable probes for diagnosis and biomarker detection by MRI

“…Westmeyer et al 19 proposed an MRI-based gene mapping methodology by using a probe HeLa cells and signal intensity was monitored over time and at increasing concentrations of the probe. A related strategy was pursued by the use of mesoporous manganese silicate coated silica NPs.…”

“…Nonetheless, the assembly/disassembly approach is one of the most frequently used to quench/activate 19 F MRI signal, because mobility restriction of the fluorine atoms is easy to achieve by encapsulation or trapping techniques and it leads to 19 F signal broadening and hence MRI signal attenuation. Yuan 52 et al designed a smart probe for imaging caspase 3/7 activity in vivo.…”

“…For the case of 1 H MRI probes there have been several reviews published in the last years which discuss smart probes for imaging in general [1][2][3][4] and also for MRI in particular, [5][6][7][8][9][10][11][12] hence we will only focus on the most recent examples, mostly from 2014 onwards. On the contrary, activatable 19 F MRI will be discussed in more detail since they have been less extensively revised. 1,13…”

Development of Timd2 as a reporter gene for MRI

“…A variety of MR reporter genes have been developed that are detectable in T 2 ‐ and T 1 ‐weighted 1 H images , in 19 F images and spectra and in 31 P spectra . Recent work has also investigated the potential of MR gene reporter systems that use hyperpolarized 13 C‐labeled metabolites and 129 Xe‐based probes , although of these only one has been demonstrated in vivo .…”