PET reporter systems for the brain

Abstract

Positron emission tomography (PET) can be used as a noninvasive method to longitudinally monitor and quantify the expression of proteins in the brain in vivo. It can be used to monitor changes in biomarkers of mental health disorders, and to assess therapeutic interventions such as stem cell and molecular genetic therapies. The utility of PET monitoring depends on the availability of a radiotracer with good central nervous system (CNS) penetration and high selectivity for the target protein. This review evaluates existing methods for the visualization of reporter proteins and/or protein function using PET imaging, focusing on engineered systems, and discusses possible approaches for future success in the development of high-sensitivity and high-specificity PET reporter systems for the brain.

Keywords: PET imaging; PET tracer; central nervous system; gene therapies; reporter probes; stem cell therapies.

Figure 1:. PET imaging: an overview.

A) Schematic representation of brain imaging with PET. A cyclotron generates radionuclides which are used for the production of PET reporter probes via one or more chemical reactions (radiochemistry). A final radiopharmaceutical formulation is prepared with a suitable vehicle after the isolation of the PET reporter probe using HPLC, and then injected into living subjects. PET imaging involves the reconstruction of the location of reporter probe within the body, via data acquisition and post-processing (see Text box 1 for details). B) Overview of the expression of reporter gene systems; three different mechanisms by which PET radiotracers image reporter gene expression based on (i) enzymes: intracellular entrapment of radiotracers after phosphorylation by enzymes encoded by reporter genes, e.g. HSV1-tk phosphorylates pyrimidine-based radiotracers, such as [¹⁸F]FIAU, trapping the phospohorylated ligand inside the cell (ii) receptors: radiotracers bind protein receptors encoded by reporter genes, e.g. the human somatostatin receptor type 2 (hSSTR2), dopamine D₂ receptor (D2R), and optogenetic-based receptors, encoded by a reporter gene (iii) transporters: accumulation of radiotracers into cells mediated by membrane transporters encoded by reporter genes, e.g. the human sodium–iodide symporter (hNIS), and human norepinephrine transporter (hNET) encoded by reporter gene.

Figure 2.. Brain PET reporter systems

(A, B) Chemogenetic - DREADD reporter system. Lenti-HA-hM4Di injections targeted to right amygdala in monkey. A: Injection location depicted on a coronal section. B: Baseline [¹¹C]DCZ binding (SUV) reveals contrast between the injection site (marked with an arrow) and the surrounding brain. (C, D) Chemogenetic - PSAM reporter system. AAV injection site in mouse left dorsal striatum (AAV-PSAM4-GlyR-EGFP). C: approximate injection location. D: PET image of mouse brain after [¹⁸F]ASEM injection, showing striatal localization of PSAM4-GlyR. (E,F) PKM2 reporter system. AAV9-EF1A-PKM2 targeted to the ventral tegmental area / substantia nigra of mouse brain. E: injection location. F: [¹⁸F]DASA-23 binding; arrow indicates region of radiotracer uptake. (G, H) ecDHFR reporter system. Recombinant ecDHFR-EGFP reporter protein was expressed in mouse somatosensory cortex after transduction with AAV-DJ. G: injection location. H: [¹⁸F]FE-TMP binding; arrow indicates area of accumulation of the radiotracer. Figures B, D, F and H were adapted from [51], [53], [56] and [57] respectively.

Figure 3:. Recovery of dopamine transporter binding six months after DA stem cell transplantation in the rat striatum in a 6-hydroxy-dopamine model of PD.

(A) A naive rat showed bilaterally symmetrical uptake of [¹⁸F]FECNT, a radioligand for the dopamine transporter. (B) Unilateral injection of a dopaminergic toxin created a hemiparkinsonian rat, which displayed negligible [¹⁸F]FECNT uptake in the lesioned striatum. (C) ES cell transplantation partially restored uptake of [¹⁸F]FECNT in the lesioned striatum. (D) Coregistered PET and MR images after ES-cell transplantation. In this example, PET imaging of the dopamine transporter was used as a PET reporter system to monitor the survival and growth of transplanted cells. Figure adapted from [71].